This is somehow an extreme exploration since it's almost unreachable on live without terrain editing or using a Z hack...6):
It's an untextured cellar type room, part of the Ratchet Inn wmo files.
As shown on the pic, you can stand on it without falling threw. (Z=[-6;0])
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If you really want to give it a look, an edited ADT that should let you slowfall down to the room is available here:
DOWNLOAD!