In Aliens, Brian 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 it was all carved out in a geological heartbeat by a monumental flood. Armed with a gas mask, Brian 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. But in Aliens, Brian's sixth wonder isn't a planet at all. Jupiter's moon Europa is a dazzling ball of ice etched with strange cracks. The patterns in the ice reveal that, far below, there is an ocean with more potentially life-giving water than all the oceans on Earth. Of all the wonders of the solar system forged by the laws of nature, there is one that stands out. In the final episode of this series, Brian reveals the greatest wonder of them all.