1
00:00:09,243 --> 00:00:12,523
[pensive music playing]

2
00:00:22,083 --> 00:00:24,443
[birds squawking]

3
00:00:27,043 --> 00:00:29,043
[Freeman] The far north of our planet

4
00:00:30,683 --> 00:00:32,843
is a land of extremes.

5
00:00:35,163 --> 00:00:36,883
[pensive music continues]

6
00:00:37,923 --> 00:00:39,483
When winter takes hold…

7
00:00:41,883 --> 00:00:43,123
[wind gusting]

8
00:00:43,203 --> 00:00:46,203
…arctic winds cause temperatures
to plummet.

9
00:00:49,843 --> 00:00:54,123
Even the hardy Alaskan brown bear
must take refuge.

10
00:00:55,243 --> 00:00:56,203
[bear grunts]

11
00:01:02,563 --> 00:01:05,203
[Freeman] Unable to endure
the freezing conditions,

12
00:01:07,083 --> 00:01:10,323
they are forced
to embark on an extraordinary journey.

13
00:01:12,643 --> 00:01:15,003
[pensive music continues]

14
00:01:15,083 --> 00:01:18,843
As the North Pole tilts
into the darkness and cold of winter…

15
00:01:24,043 --> 00:01:25,723
the bears begin climbing

16
00:01:27,043 --> 00:01:28,723
high into the mountains…

17
00:01:33,003 --> 00:01:35,443
to find shelter in caves or dens

18
00:01:36,643 --> 00:01:38,523
dug into the steep slopes.

19
00:01:41,683 --> 00:01:43,203
Hundreds of meters up.

20
00:01:45,323 --> 00:01:47,843
[ominous music playing]

21
00:01:50,283 --> 00:01:51,803
[snow crumbling]

22
00:01:54,723 --> 00:01:56,923
[wind gusting]

23
00:02:00,643 --> 00:02:03,443
[ominous music continues]

24
00:02:12,723 --> 00:02:14,923
Bears living at the ends of the Earth

25
00:02:16,803 --> 00:02:20,923
must contend with the incredible
challenges each season brings.

26
00:02:22,043 --> 00:02:23,883
[suspenseful music playing]

27
00:02:23,963 --> 00:02:24,843
To do it,

28
00:02:26,363 --> 00:02:30,283
they rely on a very special connection
to the universe,

29
00:02:31,283 --> 00:02:34,723
because written into a bear's biology

30
00:02:34,803 --> 00:02:36,883
is every critical moment…

31
00:02:36,963 --> 00:02:38,363
[whooshing]

32
00:02:38,443 --> 00:02:42,003
…of Earth's
four-and-a-half-billion-year history.

33
00:02:45,083 --> 00:02:47,563
Events of unimaginable power…

34
00:02:47,643 --> 00:02:48,763
[rumbling]

35
00:02:48,843 --> 00:02:50,083
…and destruction

36
00:02:51,523 --> 00:02:56,443
that have created a world
like nowhere else in our universe.

37
00:02:58,683 --> 00:03:01,243
[dramatic music playing]

38
00:03:18,683 --> 00:03:20,443
[ethereal music playing]

39
00:03:27,243 --> 00:03:30,043
[bear grunting]

40
00:03:30,923 --> 00:03:32,843
[sniffing]

41
00:03:33,363 --> 00:03:36,523
[Freeman] This is Luna,
a six-year-old female.

42
00:03:41,523 --> 00:03:43,923
Having reached the safety of her den…

43
00:03:46,843 --> 00:03:50,043
this will be her home
for the next six months.

44
00:03:50,563 --> 00:03:51,603
[Luna huffs]

45
00:03:58,003 --> 00:03:59,163
A cozy spot

46
00:04:00,603 --> 00:04:04,443
to see out the winter in hibernation.

47
00:04:05,443 --> 00:04:07,643
[Luna breathing heavily]

48
00:04:16,123 --> 00:04:17,163
[grunts]

49
00:04:22,683 --> 00:04:24,563
As the days get shorter…

50
00:04:27,243 --> 00:04:29,923
her body responds
to the rhythm of our planet.

51
00:04:31,323 --> 00:04:33,123
[whooshing]

52
00:04:33,203 --> 00:04:36,323
[heartbeat thumping]

53
00:04:36,403 --> 00:04:38,523
Her heart rate begins to slow…

54
00:04:38,603 --> 00:04:41,883
[heartbeat slows slightly]

55
00:04:48,323 --> 00:04:50,323
…to just eight beats per minute.

56
00:04:50,403 --> 00:04:52,643
[heartbeat slows dramatically]

57
00:04:52,723 --> 00:04:53,723
[Luna huffs]

58
00:05:04,923 --> 00:05:06,563
As she settles down…

59
00:05:08,643 --> 00:05:10,643
her body temperature drops.

60
00:05:10,723 --> 00:05:12,283
[heartbeat continues slowly]

61
00:05:27,643 --> 00:05:29,483
[ethereal music playing]

62
00:05:30,843 --> 00:05:35,923
And she falls into a deep, dreamful sleep.

63
00:05:36,003 --> 00:05:38,643
[Luna snoring]

64
00:05:38,723 --> 00:05:41,123
[ethereal music continues]

65
00:05:52,323 --> 00:05:56,803
The powerful connection
between bear and universe…

66
00:05:59,923 --> 00:06:03,563
compels her to spend
the harsh winter months underground.

67
00:06:03,643 --> 00:06:05,323
[pensive music playing]

68
00:06:08,003 --> 00:06:12,123
Blissfully unaware
of the long journey ahead.

69
00:06:17,203 --> 00:06:19,163
Because outside the den…

70
00:06:19,243 --> 00:06:21,123
[whooshing]

71
00:06:22,523 --> 00:06:24,283
…Earth is on the move.

72
00:06:24,363 --> 00:06:26,523
[pensive music continues]

73
00:06:34,323 --> 00:06:38,443
Every day,
the planet spins once on its axis.

74
00:06:38,523 --> 00:06:39,923
[whooshing]

75
00:06:40,003 --> 00:06:42,123
[pensive music continues]

76
00:06:55,363 --> 00:07:00,123
Each month, the Moon completes
one full circuit around the Earth.

77
00:07:07,083 --> 00:07:08,723
And as the months pass,

78
00:07:09,763 --> 00:07:13,363
the Earth and the Moon orbit the Sun.

79
00:07:16,123 --> 00:07:18,563
[whooshing]

80
00:07:19,243 --> 00:07:21,363
During the time she'll spend asleep,

81
00:07:22,363 --> 00:07:26,563
Luna will travel 470 million kilometers.

82
00:07:26,643 --> 00:07:29,083
[pensive music continues]

83
00:07:33,003 --> 00:07:34,963
An epic cosmic voyage

84
00:07:36,683 --> 00:07:38,683
halfway around the Sun.

85
00:07:42,203 --> 00:07:44,723
[calm music playing]

86
00:07:47,603 --> 00:07:48,723
And this year,

87
00:07:49,603 --> 00:07:52,243
she has some new traveling companions.

88
00:07:52,923 --> 00:07:55,523
[cub moaning]

89
00:07:55,603 --> 00:07:58,243
While in and out of sleep…

90
00:07:58,323 --> 00:07:59,483
[cub yowls]

91
00:07:59,563 --> 00:08:01,683
…Luna has given birth…

92
00:08:04,083 --> 00:08:05,923
to two little bears.

93
00:08:12,363 --> 00:08:16,523
The cubs will eat and sleep
in the den until springtime.

94
00:08:19,643 --> 00:08:21,443
But two more mouths to feed

95
00:08:23,363 --> 00:08:26,043
is taking its toll on Luna's body.

96
00:08:27,403 --> 00:08:29,443
[whooshing]

97
00:08:33,163 --> 00:08:35,843
Fat cells beneath her skin begin to shrink

98
00:08:37,483 --> 00:08:40,843
as her energy stores
are used up producing milk.

99
00:08:44,963 --> 00:08:47,443
[pensive music playing]

100
00:08:48,963 --> 00:08:51,003
After five months underground,

101
00:08:51,883 --> 00:08:55,443
Luna has already lost
more than a quarter of her body weight.

102
00:08:55,523 --> 00:08:58,123
[cubs grunting softly]

103
00:08:59,363 --> 00:09:01,083
[wind howling]

104
00:09:01,163 --> 00:09:04,483
And with outside temperatures
still locked below zero…

105
00:09:06,603 --> 00:09:09,843
Luna urgently needs spring to arrive

106
00:09:10,683 --> 00:09:14,043
if she is to continue feeding
her newborn cubs.

107
00:09:18,763 --> 00:09:20,923
[cub snoring]

108
00:09:22,123 --> 00:09:23,003
Luckily,

109
00:09:24,923 --> 00:09:26,403
Earth has a happy quirk.

110
00:09:29,843 --> 00:09:31,523
It's tilted on its side.

111
00:09:31,603 --> 00:09:34,283
[calm music playing]

112
00:09:35,963 --> 00:09:38,843
And so, as it moves through space,

113
00:09:39,763 --> 00:09:42,283
at different times of the year,

114
00:09:42,363 --> 00:09:45,443
different parts
of the planet lean toward the Sun,

115
00:09:47,323 --> 00:09:49,683
causing the seasons to turn.

116
00:10:00,403 --> 00:10:02,043
Which means, in Alaska,

117
00:10:03,243 --> 00:10:05,243
something remarkable is happening.

118
00:10:05,323 --> 00:10:07,483
[ethereal music playing]

119
00:10:12,923 --> 00:10:14,803
With each passing day,

120
00:10:14,883 --> 00:10:17,043
the Sun gets higher in the sky.

121
00:10:20,883 --> 00:10:23,723
Its warming rays transforming
the landscape.

122
00:10:34,123 --> 00:10:37,963
Just when Luna and her cubs need it most.

123
00:10:42,563 --> 00:10:46,123
Nerve endings in her skin detect
the warmth of the Sun.

124
00:10:50,203 --> 00:10:52,723
[majestic music playing]

125
00:10:53,923 --> 00:10:55,603
And as the days get longer…

126
00:10:55,683 --> 00:10:57,323
[Luna snorting]

127
00:10:59,523 --> 00:11:01,083
[whooshing]

128
00:11:01,163 --> 00:11:04,043
…specialized cells in the back of her eye

129
00:11:05,523 --> 00:11:09,043
sense the tiny increases in light levels.

130
00:11:09,123 --> 00:11:11,443
[majestic music continues]

131
00:11:14,723 --> 00:11:17,003
These secret messages,

132
00:11:17,083 --> 00:11:19,243
direct from our nearest star…

133
00:11:22,443 --> 00:11:26,323
tell Luna it's time
to wake from hibernation.

134
00:11:27,963 --> 00:11:29,203
[Luna groans loudly]

135
00:11:35,763 --> 00:11:37,363
[yawning]

136
00:11:40,763 --> 00:11:45,203
As they signal the gentle turning
of our planet's seasons.

137
00:11:45,923 --> 00:11:46,963
[snorts]

138
00:11:47,483 --> 00:11:50,403
[groans, snorts]

139
00:11:50,483 --> 00:11:52,123
[whooshing]

140
00:11:54,363 --> 00:11:56,043
But out in the solar system…

141
00:11:58,243 --> 00:12:00,083
seasons like Earth's

142
00:12:01,923 --> 00:12:03,243
are far from typical.

143
00:12:03,323 --> 00:12:05,683
[ominous music playing]

144
00:12:09,683 --> 00:12:12,203
Our nearest neighbor, Venus…

145
00:12:12,723 --> 00:12:13,843
[whooshing]

146
00:12:14,963 --> 00:12:16,723
…is much closer to the Sun

147
00:12:17,683 --> 00:12:20,403
and so is much hotter than Earth.

148
00:12:20,483 --> 00:12:21,403
[whooshing]

149
00:12:21,483 --> 00:12:23,123
[fizzling]

150
00:12:25,203 --> 00:12:28,163
And because the planet has no tilt,

151
00:12:29,003 --> 00:12:31,123
it has virtually no seasons.

152
00:12:32,003 --> 00:12:34,163
[ominous music continues]

153
00:12:35,523 --> 00:12:39,443
Temperatures remain searingly hot
all year round.

154
00:12:44,323 --> 00:12:45,603
Further away,

155
00:12:46,123 --> 00:12:47,523
out past Earth…

156
00:12:47,603 --> 00:12:48,883
[whooshing]

157
00:12:49,643 --> 00:12:50,963
…is Mars,

158
00:12:52,923 --> 00:12:55,723
whose orbit causes its distance
from the Sun

159
00:12:56,243 --> 00:12:58,483
to vary dramatically through the year,

160
00:13:00,203 --> 00:13:02,003
making seasons on Mars…

161
00:13:02,523 --> 00:13:03,563
[ice crackling]

162
00:13:03,643 --> 00:13:06,483
…far more extreme than our own.

163
00:13:09,243 --> 00:13:13,923
During winter, over a quarter
of the planet's atmosphere freezes solid.

164
00:13:15,923 --> 00:13:17,083
While in summer,

165
00:13:18,163 --> 00:13:20,483
when Mars passes closer to the Sun,

166
00:13:21,283 --> 00:13:24,323
the increased heat creates dust storms

167
00:13:24,403 --> 00:13:29,323
so severe
they can engulf the entire planet.

168
00:13:29,403 --> 00:13:31,203
[suspenseful music playing]

169
00:13:31,283 --> 00:13:33,323
[whooshing]

170
00:13:33,403 --> 00:13:34,843
But beyond Mars,

171
00:13:36,523 --> 00:13:37,723
past Jupiter…

172
00:13:37,803 --> 00:13:39,643
[whooshing]

173
00:13:41,123 --> 00:13:42,043
…and Saturn…

174
00:13:44,843 --> 00:13:46,563
[whooshing]

175
00:13:48,123 --> 00:13:51,043
…three billion kilometers from the Sun,

176
00:13:52,643 --> 00:13:57,243
Uranus has the most extreme seasons
in the solar system.

177
00:13:59,883 --> 00:14:03,283
This planet spins on its side.

178
00:14:04,323 --> 00:14:06,283
So as it orbits,

179
00:14:06,363 --> 00:14:08,523
the summer side faces the Sun…

180
00:14:09,043 --> 00:14:11,323
[wind gusting]

181
00:14:12,083 --> 00:14:17,123
…while the winter side
won't see daylight for 42 years.

182
00:14:21,323 --> 00:14:24,563
[whooshing]

183
00:14:24,643 --> 00:14:27,123
[ethereal music playing]

184
00:14:29,443 --> 00:14:32,843
Only Earth has seasons

185
00:14:34,483 --> 00:14:37,483
that, even at the extremes of our planet,

186
00:14:38,243 --> 00:14:40,483
allow life a fighting chance.

187
00:14:47,043 --> 00:14:49,443
[calm music playing]

188
00:14:54,563 --> 00:14:56,163
With the arrival of spring,

189
00:14:57,323 --> 00:14:59,643
Luna's family emerge from the den.

190
00:15:01,323 --> 00:15:03,843
[cubs grunting softly]

191
00:15:06,323 --> 00:15:11,043
The cubs can now enjoy
their first moments in the Sun.

192
00:15:13,923 --> 00:15:15,363
[cubs continue grunting]

193
00:15:19,923 --> 00:15:20,923
[cub yowls]

194
00:15:21,003 --> 00:15:23,203
[calm music continues]

195
00:15:35,923 --> 00:15:40,403
But the turning seasons mean the cubs
must now set off on a daunting journey.

196
00:15:48,323 --> 00:15:49,803
Because to find food,

197
00:15:50,883 --> 00:15:53,523
Luna must lead them back to the lowlands

198
00:15:54,803 --> 00:15:56,803
on the other side of the mountain.

199
00:16:02,003 --> 00:16:04,403
[pensive music playing]

200
00:16:04,483 --> 00:16:05,483
[Luna grunts]

201
00:16:13,763 --> 00:16:16,043
It's a grueling climb for little legs.

202
00:16:16,803 --> 00:16:19,163
[majestic music playing]

203
00:16:23,883 --> 00:16:27,363
With hundreds of meters
of ice and rock to overcome.

204
00:16:27,443 --> 00:16:29,243
[intriguing music playing]

205
00:16:29,323 --> 00:16:30,483
[cubs grunting]

206
00:16:50,603 --> 00:16:53,243
[calm music playing]

207
00:17:01,483 --> 00:17:02,363
[Luna groaning]

208
00:17:13,123 --> 00:17:14,043
[cub whines]

209
00:17:21,083 --> 00:17:25,323
Once they're over the summit,
the hardest part of the trek is over.

210
00:17:25,403 --> 00:17:27,763
[calm music continues]

211
00:17:28,883 --> 00:17:30,043
[Luna groans]

212
00:17:36,803 --> 00:17:39,963
But survival is far from guaranteed.

213
00:17:43,603 --> 00:17:45,963
To make it through the rest of the year,

214
00:17:47,283 --> 00:17:48,883
the family will rely…

215
00:17:48,963 --> 00:17:49,963
[whooshing]

216
00:17:50,043 --> 00:17:52,043
…on Luna's instinctive understanding

217
00:17:53,243 --> 00:17:54,683
of the seasons ahead.

218
00:17:54,763 --> 00:17:57,363
[ethereal music playing]

219
00:17:58,963 --> 00:18:00,323
Each one…

220
00:18:02,683 --> 00:18:05,523
a result of Earth's yearly journey
through space.

221
00:18:12,483 --> 00:18:13,883
A journey that began…

222
00:18:14,523 --> 00:18:15,883
[whooshing]

223
00:18:15,963 --> 00:18:19,003
…before the Earth even existed.

224
00:18:23,323 --> 00:18:26,123
[ethereal music playing]

225
00:18:33,683 --> 00:18:39,643
A time when our solar system
was little more than ash.

226
00:18:43,163 --> 00:18:44,163
No Sun,

227
00:18:44,923 --> 00:18:45,763
no planets,

228
00:18:46,483 --> 00:18:47,643
and no moons.

229
00:18:50,403 --> 00:18:53,963
Just a vast cloud of gas and dust.

230
00:18:54,043 --> 00:18:56,443
[mysterious music playing]

231
00:19:07,363 --> 00:19:11,003
Then, in a distant solar system…

232
00:19:11,083 --> 00:19:13,363
[ominous music playing]

233
00:19:13,443 --> 00:19:14,963
…a star explodes.

234
00:19:15,043 --> 00:19:16,883
[rumbling in distance]

235
00:19:16,963 --> 00:19:21,843
A supernova that sends
a shock wave across the galaxy.

236
00:19:21,923 --> 00:19:24,123
[dramatic music playing]

237
00:19:27,603 --> 00:19:30,003
[rumbling growing louder]

238
00:19:34,243 --> 00:19:36,003
[dramatic music crescendos]

239
00:19:41,323 --> 00:19:46,323
On impact, the shockwave
causes the cloud to fall in on itself.

240
00:19:46,403 --> 00:19:48,923
[ethereal music playing]

241
00:19:51,723 --> 00:19:53,483
And begin to rotate.

242
00:19:53,563 --> 00:19:55,923
[whooshing]

243
00:20:00,483 --> 00:20:02,683
As it spins faster and faster,

244
00:20:04,083 --> 00:20:08,323
the cloud flattens out into a disc.

245
00:20:10,243 --> 00:20:13,243
A maelstrom of gas and rock.

246
00:20:16,643 --> 00:20:19,443
In the center, under intense pressure…

247
00:20:21,923 --> 00:20:23,003
[crashing]

248
00:20:23,083 --> 00:20:24,523
…the Sun is born.

249
00:20:28,123 --> 00:20:31,883
While further out,
rock collides with rock…

250
00:20:34,123 --> 00:20:36,243
and our planet begins to grow.

251
00:20:41,123 --> 00:20:43,523
Until after 20 million years,

252
00:20:44,923 --> 00:20:46,803
Earth is fully formed.

253
00:20:49,403 --> 00:20:52,763
Still circling the Sun like the cloud
from which it grew.

254
00:20:52,843 --> 00:20:55,603
[dramatic music playing]

255
00:20:58,963 --> 00:21:02,323
Orbiting once every 365 days,

256
00:21:05,563 --> 00:21:07,483
which is how Earth got its year.

257
00:21:14,283 --> 00:21:16,643
But at this point in the planet's story,

258
00:21:17,803 --> 00:21:19,563
it's a year without seasons.

259
00:21:20,963 --> 00:21:21,843
[whooshing]

260
00:21:21,923 --> 00:21:24,043
[rumbling]

261
00:21:24,123 --> 00:21:26,763
Because the chaos that created them…

262
00:21:29,243 --> 00:21:31,123
is still to come.

263
00:21:32,243 --> 00:21:35,603
[insects, birds chirping]

264
00:21:38,643 --> 00:21:41,523
[calm music playing]

265
00:21:41,603 --> 00:21:42,723
In Alaska,

266
00:21:43,683 --> 00:21:45,403
the vibrancy of spring…

267
00:21:45,483 --> 00:21:46,563
[cub whining]

268
00:21:46,643 --> 00:21:49,323
…has given way to lazy summer days.

269
00:22:01,523 --> 00:22:03,923
The cubs are now five months old.

270
00:22:04,003 --> 00:22:05,083
[cub whining]

271
00:22:05,163 --> 00:22:06,563
[playful music playing]

272
00:22:06,643 --> 00:22:08,163
And still learning…

273
00:22:10,443 --> 00:22:11,603
how to be bears.

274
00:22:12,243 --> 00:22:13,123
[cub yelping]

275
00:22:25,043 --> 00:22:27,123
Because they're growing so quickly,

276
00:22:28,083 --> 00:22:30,003
the cubs are constantly hungry.

277
00:22:36,563 --> 00:22:38,843
And while it might look idyllic…

278
00:22:41,083 --> 00:22:44,523
summer in Alaska is a time of hardship.

279
00:22:44,603 --> 00:22:47,603
[pensive music playing]

280
00:22:51,403 --> 00:22:53,243
[sniffs]

281
00:22:53,323 --> 00:22:54,203
[whines]

282
00:22:54,723 --> 00:22:57,563
Without a decent meal to be found
in the meadows,

283
00:22:58,083 --> 00:23:00,243
the bears must make do with grass.

284
00:23:11,283 --> 00:23:16,283
Which means the cubs still rely
on their mother's milk

285
00:23:16,363 --> 00:23:17,963
for the bulk of their food.

286
00:23:20,163 --> 00:23:21,323
[Luna groaning]

287
00:23:28,283 --> 00:23:30,803
[pensive music continues]

288
00:23:32,483 --> 00:23:35,483
[cubs purring]

289
00:23:35,563 --> 00:23:39,843
But Luna hasn't eaten properly
since before she went into hibernation,

290
00:23:41,603 --> 00:23:44,083
so she's running on empty.

291
00:23:44,163 --> 00:23:46,563
[Luna breathing heavily]

292
00:23:53,123 --> 00:23:56,083
But nearby, out in space,

293
00:23:57,563 --> 00:23:59,203
help is on the way.

294
00:24:02,403 --> 00:24:04,483
[whooshing]

295
00:24:05,923 --> 00:24:10,443
Because today,
our planet has a constant companion.

296
00:24:10,963 --> 00:24:12,963
[dramatic music playing]

297
00:24:13,043 --> 00:24:14,083
The Moon.

298
00:24:16,363 --> 00:24:18,323
Locked in orbit around the Earth,

299
00:24:19,763 --> 00:24:22,363
it's our nearest celestial neighbor.

300
00:24:23,963 --> 00:24:28,003
The largest and brightest object
in the night sky.

301
00:24:32,163 --> 00:24:35,563
And because the Moon
is very big and very close…

302
00:24:37,803 --> 00:24:40,523
it exerts a powerful gravitational pull.

303
00:24:40,603 --> 00:24:42,763
[dramatic music continues]

304
00:24:47,923 --> 00:24:51,523
Strong enough to influence our world.

305
00:24:56,123 --> 00:24:59,203
It lifts the oceans away
from the planet's surface…

306
00:25:05,923 --> 00:25:07,483
[water rippling]

307
00:25:07,563 --> 00:25:09,723
…creating Earth's tides.

308
00:25:16,003 --> 00:25:18,723
And on a few days
around the full and new Moon…

309
00:25:20,843 --> 00:25:25,443
the lowest summer tides reveal
an entirely new landscape.

310
00:25:25,523 --> 00:25:27,803
[birds squawking]

311
00:25:29,803 --> 00:25:32,843
Mudflats extend hundreds of meters
from the shore.

312
00:25:38,243 --> 00:25:39,083
[Luna grunts]

313
00:25:39,163 --> 00:25:42,843
Allowing hungry bears access
to buried treasure.

314
00:25:44,483 --> 00:25:47,083
[calm music playing]

315
00:25:50,603 --> 00:25:54,523
At just the right time
for Luna and her young family.

316
00:25:54,603 --> 00:25:56,043
[sniffing]

317
00:26:02,523 --> 00:26:04,763
But finding what's hidden in the mud

318
00:26:05,523 --> 00:26:08,243
is a job best left to the grown-ups.

319
00:26:09,723 --> 00:26:11,843
[sniffing]

320
00:26:17,123 --> 00:26:21,723
Luna's sense of smell
is 2,000 times better than our own.

321
00:26:24,523 --> 00:26:26,923
Perfect for detecting tasty morsels.

322
00:26:31,243 --> 00:26:32,723
Soft-shelled clams.

323
00:26:32,803 --> 00:26:35,483
[playful music playing]

324
00:26:53,723 --> 00:26:56,243
She's so skilled at prizing them apart,

325
00:26:57,443 --> 00:27:00,243
she can get through almost one a minute.

326
00:27:12,883 --> 00:27:15,123
[sniffing]

327
00:27:20,483 --> 00:27:24,043
The clams are a welcome supplement
to her meager summer diet.

328
00:27:31,763 --> 00:27:34,643
Vital energy
she can pass along to the cubs.

329
00:27:34,723 --> 00:27:38,643
[cubs purring]

330
00:27:39,843 --> 00:27:43,403
These gifts, delivered to her by the Moon,

331
00:27:43,923 --> 00:27:46,803
will help Luna sustain her family
through summer.

332
00:27:49,923 --> 00:27:52,523
But the bear's relationship with the Moon…

333
00:27:52,603 --> 00:27:53,643
[whooshing]

334
00:27:53,723 --> 00:27:55,403
[ethereal music playing]

335
00:27:55,483 --> 00:27:58,483
…runs much, much deeper.

336
00:27:58,563 --> 00:28:00,563
-[Luna groans]
-[whooshing]

337
00:28:01,443 --> 00:28:02,443
[growls]

338
00:28:03,283 --> 00:28:08,323
Because it was during the Moon's formation
that Earth got its seasons.

339
00:28:13,923 --> 00:28:16,083
Not long after it formed,

340
00:28:17,003 --> 00:28:18,803
our planet gained a companion.

341
00:28:22,243 --> 00:28:25,123
[ethereal music playing]

342
00:28:31,043 --> 00:28:32,843
But it wasn't the Moon.

343
00:28:34,963 --> 00:28:39,123
It was another planet
that formed in Earth's orbit.

344
00:28:44,563 --> 00:28:46,923
A sibling called Theia.

345
00:28:48,723 --> 00:28:50,843
It was roughly the size of Mars.

346
00:28:52,803 --> 00:28:56,723
-[lava bubbling, exploding]
-A vast ball of rock and lava.

347
00:28:59,483 --> 00:29:01,243
Throughout their childhood,

348
00:29:03,083 --> 00:29:05,163
Earth and Theia lived in harmony.

349
00:29:05,243 --> 00:29:07,523
[ominous music playing]

350
00:29:11,363 --> 00:29:16,323
Until gravitational disturbances
in the young solar system…

351
00:29:19,723 --> 00:29:23,443
set the two planets on a collision course.

352
00:29:28,923 --> 00:29:30,083
In this moment…

353
00:29:30,843 --> 00:29:32,003
[rumbling]

354
00:29:32,083 --> 00:29:35,083
…Earth's future hangs in the balance.

355
00:29:38,323 --> 00:29:40,723
[rumbling growing louder]

356
00:29:40,803 --> 00:29:42,963
[ominous music intensifies]

357
00:29:45,323 --> 00:29:46,963
[crashing]

358
00:29:54,043 --> 00:29:56,083
Theia is obliterated…

359
00:29:59,523 --> 00:30:02,443
as the impact melts both worlds.

360
00:30:12,323 --> 00:30:14,723
[ethereal music playing]

361
00:30:29,163 --> 00:30:31,443
For an instant in cosmic time…

362
00:30:34,603 --> 00:30:36,523
our planet had two moons.

363
00:30:41,043 --> 00:30:43,723
[fire roaring, crackling]

364
00:30:46,403 --> 00:30:53,203
But then gravity dragged one of them
back down to be consumed by Earth.

365
00:30:55,003 --> 00:30:57,403
[ethereal music continues]

366
00:31:06,763 --> 00:31:09,923
Leaving only the moon we know today.

367
00:31:17,083 --> 00:31:21,203
And the collision transformed
our planet forever.

368
00:31:21,723 --> 00:31:23,283
[whooshing]

369
00:31:23,363 --> 00:31:26,123
[calm music playing]

370
00:31:28,323 --> 00:31:30,643
The impact knocked it over to one side

371
00:31:31,643 --> 00:31:35,083
so that today it spins
with a tilt of 23 degrees.

372
00:31:41,003 --> 00:31:43,803
So, as it moves around in its orbit,

373
00:31:45,643 --> 00:31:47,083
first one hemisphere

374
00:31:47,763 --> 00:31:51,683
then the other is angled toward the Sun…

375
00:31:55,043 --> 00:31:57,923
giving Earth its seasons.

376
00:31:58,003 --> 00:32:00,563
[dramatic music playing]

377
00:32:10,723 --> 00:32:12,803
[owl hooting]

378
00:32:19,643 --> 00:32:21,963
As summer draws to a close,

379
00:32:22,723 --> 00:32:25,243
the longer nights are a sign

380
00:32:25,323 --> 00:32:28,323
that the harsh Alaskan winter
is fast approaching.

381
00:32:32,603 --> 00:32:35,243
The cubs are now seven months old.

382
00:32:41,163 --> 00:32:45,683
And Luna knows that if they are to survive
their long winter sleep,

383
00:32:46,403 --> 00:32:49,443
the family must build up
their fat reserves.

384
00:32:50,803 --> 00:32:51,923
[Luna groans]

385
00:33:03,843 --> 00:33:04,923
[moaning]

386
00:33:06,043 --> 00:33:08,283
But the Moon is a constant reminder…

387
00:33:09,923 --> 00:33:12,443
[bellowing]

388
00:33:12,963 --> 00:33:14,443
…help is on the way.

389
00:33:19,843 --> 00:33:23,003
A thousand kilometers
off the coast of Alaska…

390
00:33:25,003 --> 00:33:28,523
another creature is tuned in
to Earth's seasonal rhythm.

391
00:33:28,603 --> 00:33:30,363
[calm music playing]

392
00:33:32,243 --> 00:33:34,243
Sockeye salmon.

393
00:33:36,803 --> 00:33:39,123
After two years roaming the Pacific Ocean…

394
00:33:40,923 --> 00:33:42,283
[ethereal tinkling]

395
00:33:42,803 --> 00:33:44,163
…they've reached maturity.

396
00:33:46,123 --> 00:33:48,443
And now, the changing day length…

397
00:33:48,523 --> 00:33:49,563
[whooshing]

398
00:33:49,643 --> 00:33:52,723
…triggers hormones
to be released into their blood.

399
00:33:55,763 --> 00:33:59,123
Chemical signals
that alter their behavior.

400
00:33:59,203 --> 00:34:02,123
[whooshing]

401
00:34:03,403 --> 00:34:06,043
[joyful music playing]

402
00:34:08,123 --> 00:34:09,483
Calling them home

403
00:34:10,963 --> 00:34:13,843
to breed in the rivers
where they were born.

404
00:34:20,643 --> 00:34:22,083
[joyful music crescendos]

405
00:34:26,523 --> 00:34:28,203
[pensive music playing]

406
00:34:28,283 --> 00:34:29,483
[groans]

407
00:34:34,323 --> 00:34:36,123
Luna's exhausted and hungry,

408
00:34:37,163 --> 00:34:41,643
but still she leads her cubs
to the river's edge.

409
00:34:42,323 --> 00:34:45,043
[grunting]

410
00:34:54,123 --> 00:34:59,043
Because she senses something magical
is about to happen.

411
00:34:59,123 --> 00:35:00,363
[cub grunts]

412
00:35:04,123 --> 00:35:09,003
Today, the unique clockwork of the seasons

413
00:35:09,083 --> 00:35:12,443
will deliver a vital lifeline
to the bears.

414
00:35:14,923 --> 00:35:17,243
[dramatic music playing]

415
00:35:33,563 --> 00:35:37,723
The Earth has reached a pivotal position
in its journey around the Sun.

416
00:35:41,763 --> 00:35:44,283
And now the Moon raises the tide.

417
00:35:46,603 --> 00:35:47,443
[Luna grunts]

418
00:35:50,043 --> 00:35:55,323
Providing the final push
that drives the salmon into the rivers.

419
00:35:56,523 --> 00:35:58,723
[Luna breathing heavily]

420
00:36:06,323 --> 00:36:08,803
[pensive music playing]

421
00:36:21,323 --> 00:36:22,243
[cub whines]

422
00:36:29,563 --> 00:36:31,923
[playful music playing]

423
00:36:39,123 --> 00:36:40,443
[Luna groans]

424
00:36:40,523 --> 00:36:45,723
At long last,
the cubs get their first taste of salmon.

425
00:36:51,203 --> 00:36:55,003
And although a single fish
will not see the bears through winter,

426
00:36:55,963 --> 00:37:01,443
it's the cubs' first real meal
in nearly eight months of life.

427
00:37:04,083 --> 00:37:06,443
[playful music continues]

428
00:37:15,363 --> 00:37:17,083
[Luna huffing]

429
00:37:18,083 --> 00:37:21,923
The arrival of the first salmon
is a relief to Luna.

430
00:37:24,763 --> 00:37:26,643
Because experience has told her

431
00:37:27,763 --> 00:37:31,243
there will soon be
much richer pickings upstream.

432
00:37:46,483 --> 00:37:47,563
[whooshing]

433
00:37:50,763 --> 00:37:52,763
As the days grow ever shorter,

434
00:37:54,443 --> 00:37:58,323
the very same hormones
that set the salmon off on their journey

435
00:37:59,243 --> 00:38:01,443
now dramatically transform them.

436
00:38:02,443 --> 00:38:04,243
[whooshing]

437
00:38:06,243 --> 00:38:08,923
[calm music playing]

438
00:38:09,003 --> 00:38:10,603
Their heads change shape.

439
00:38:13,043 --> 00:38:15,723
And their bodies blush bright red.

440
00:38:19,083 --> 00:38:21,003
As in their millions,

441
00:38:21,523 --> 00:38:25,963
they surge upriver
into their spawning grounds.

442
00:38:26,043 --> 00:38:28,443
[calm music continues]

443
00:38:36,163 --> 00:38:40,443
And the final piece
of the bears' story falls into place.

444
00:38:49,163 --> 00:38:52,563
A story as old as the planet itself.

445
00:38:53,643 --> 00:38:56,243
[dramatic music playing]

446
00:39:02,723 --> 00:39:04,323
The formation of the Earth…

447
00:39:09,803 --> 00:39:12,083
the collision that created the Moon…

448
00:39:15,043 --> 00:39:18,883
and our planet's unique relationship
with its moon and star…

449
00:39:25,443 --> 00:39:31,563
have combined to create a world
like nowhere else in the universe.

450
00:39:33,963 --> 00:39:35,643
A Goldilocks planet.

451
00:39:38,643 --> 00:39:42,723
The only place we know
that's just right for life.

452
00:39:42,803 --> 00:39:46,243
[dramatic music crescendos]

453
00:40:04,363 --> 00:40:05,403
And for Luna,

454
00:40:06,043 --> 00:40:09,043
four and a half billion years
of cosmic history

455
00:40:10,963 --> 00:40:14,043
is now distilled into this moment.

456
00:40:21,803 --> 00:40:27,243
The annual convergence of salmon and bear.

457
00:40:38,203 --> 00:40:40,443
And the precious opportunity

458
00:40:42,203 --> 00:40:43,483
to feed her family.

459
00:40:47,043 --> 00:40:49,323
[dramatic music playing]

460
00:40:51,163 --> 00:40:55,123
Luna and her cubs now have what they need
to make it through winter.

461
00:40:57,443 --> 00:41:02,003
And while ultimately, their future
on our changing planet is uncertain,

462
00:41:03,363 --> 00:41:06,523
for this year,
the salmon will sustain them…

463
00:41:10,443 --> 00:41:13,163
as our planet continues
on this relentless voyage…

464
00:41:13,243 --> 00:41:15,643
[Luna bellowing]

465
00:41:15,723 --> 00:41:17,043
…around the Sun.

466
00:41:18,043 --> 00:41:19,763
[dramatic music crescendos]

467
00:41:23,963 --> 00:41:26,603
[dramatic music playing]

468
00:41:26,683 --> 00:41:29,763
Next in the story of our universe,

469
00:41:31,203 --> 00:41:34,763
the ocean wanderer
whose life cycle reveals

470
00:41:35,443 --> 00:41:39,363
how everything in the universe was made.

471
00:41:41,163 --> 00:41:43,443
Because in every turtle's egg

472
00:41:44,323 --> 00:41:46,723
are atoms from The Big Bang…

473
00:41:47,523 --> 00:41:48,963
[ethereal tinkling]

474
00:41:49,043 --> 00:41:52,603
…forged as stars die.

475
00:41:53,563 --> 00:41:54,923
[whooshing]

476
00:41:55,003 --> 00:41:56,123
[exploding]

477
00:41:56,203 --> 00:41:59,043
But how are stars and planets created?

478
00:42:00,883 --> 00:42:07,123
And how do the same ingredients
transform into living things?

479
00:42:14,483 --> 00:42:18,403
[dramatic music playing]