Several of the underground caves house some buildings similar to what we'd find in Hadriel, but far more primitive- no appliances, windows, or even hinges on the doors would be there, but the overall circular and shell-like style would remain the same. Some of these houses look as if they were occupied and then evacuated quickly- there's fish bones, crude tools used for cooking, and various shell fragments. There is a very dim bio-luminescent lighting coming off of the walls of the cave. Any trace of a living civilization however, is long gone.

There may be more details on this later, but at the surface level, this is what Glacius and anyone else exploring would find!