Titan

Titan was the largest moon of Saturn, the sixth planet in the Sol system. It is also the second largest satellite in the Sol system, and the only such body to have a dense atmosphere, and is larger than some planetary bodies in the solar system.

By the mid-21st century, Humans had established a series of domed settlements on Titan, including Christopher's Landing. This colony became a safe haven as Earth was ravaged by World War III.

The colony continued into the 26th century, by which time the atmosphere had been modified to class M status. At the time, the colonists began a geothermal heating project, tapping into the interior heat of the moon. It was hoped that before too long the atmosphere of Titan would be warmed to the point that the domes would no longer be needed.