[PA Engineer]

To me, it looks like a massive sinkhole, something that you'd see on the surface that was caused by a huge subterranean collapse.

I agree and a collapse may be consistent with a lack of any major seismic activity.

[TXnMA]

Comparison of the before and after images in #45 shows no change in terrain topography that I could detect.

The sole difference I see is the change in albedo in the area surrounding the north entrances — as if someone blew soot on the whole area.