Crypt

Beneath the cathedral, there was a crypt, which was constructed by knight that went on record to have built it all in one night. Hundreds of years later, when Iris Wildthyme, Panda, and the verger entered the crypt, Iris thought that the knight must've been aided by time dilation.

There were stalactites descending from the ceiling, but when a "ghost" caused the cathedral to tremble, the stalactites crumbled. There were also piratical memento mori slabs around the crypt.

There was an alien spaceship buried there, which was possibly placed there by Iris and Panda, as well as the Celestial Omnibus being buried behind a wall. (PROSE: Iris Wildthyme and the Unholy Ghost)