Level 20: "The World Is Hollow"
(Created by Adam Ashwell. The title is a reference to the title of an early Star Trek: The Original Series episode, "For The World Is Hollow, And I Have Touched The Sky", featuring a multi-generational colony ship carved out of an asteroid, the inhabitants of which had forgotten that it was an asteroid. Resemblances to the Marathon aside, I thought the title was apt for a level set inside of a hollow world like this.)
You find yourself again at the top of the mountain in the world with the strange sky, the door still locked, the only place to go the well at the foot of the mountain, which leads to the even stranger room in space, where a terminal awaits you.
[L20-T0-Unfinished]
You teleport out to Level 21: "S'pht'ia".