05:03
The Water Planet: Early Earth
At one time, Earth was a very different planet than it is today. Its atmosphere, in particular, bore very little resemblance to the one that we depend on for the air we breathe. It probably consisted primarily of carbon dioxide and nitrogen gas.
03:48
The Water Planet: Big Bang, The
The "Big Bang" is the commonly accepted theory of the origin of the universe. Most scientists believe this cataclysmic explosion took place about fourteen billion years ago. It is widely thought that the universe has been expanding ever since.