So I was messing around one day, trying to survive, when I found a small town, full of irradiated wanderers. What was even more interesting was that there was a two tile church that didn't look like any I'd seen before (In this game), and a bunch of strange craters. About to die, I decided to scrap the character and go into debug mode to check it out (Yes, I know, I'm a cheater. But who really cares in a single player game?).
NOTE: I'm pretty sure this is something spoiler heavy, which I didn't really mind ruining for myself, but if you do value that spoiler free play-through, ignore the rest of this.
Using reveal map, I looked around and saw a huge sewer system underneath, as well as two "Caverns" on z-1. On z-2 and z-3, there was a mess of yellow and white symbols (Work in progress?) which were labelled as "Vault library" and "Hospital" and "Housing" etc. I noticed there were two groups of NPCS, both told me to get out, in one sentence, with one option. There was a ton of loot in there, plenty of clothes on the ground, and lots of geiger counters. The vault seems to be static, not randomly generated, though I'm not 100% sure. If you want to spawn it in debug mode in the overmap editor, it's labelled "necropolis".
Does anyone have a clue as to what this is?