Brian Cox descends to the bottom of the Pacific in a submarine to witness the extraordinary life forms that survive in the cold, black waters. All life on Earth needs water so the search for aliens in the solar system has followed the search for water.

Soaring above the dramatic Scablands of the United States, Brian discovers how the same landscape has been found on Mars, and armed with a gas mask, he enters a cave in Mexico where bacteria breathe toxic gas and leak concentrated acid. Yet relatives of these creatures could be surviving in newly-discovered caves on Mars.