What built this prison in the 1.7 DW20 pack?

I've been playing the new Direwolf20 pack for about a day now, and I happened upon this sort of abandoned prison while strip mining. Note that, as of now, DungeonPack and Walled City Generator are not in the DW20 1.7 pack, so they can't be what made this:

That's all there is to this, just a series of 'cell blocks' with wooden doors connecting them, each containing a couple of cells with iron doors. It was also filled with silverfish, though I can't find any kind of spawner.

I've been poking around to see if there's any loot to be had, because prisoners are known to be good at hiding stuff - but I'm coming up empty. I've been mining out the cell walls, ceilings and floors, as well as the halls.

My question is, what generated this? Is it something new in Vanilla 1.7, or did one of the mods in the new DW20 pack cause it to generate? If I can figure that much out, I can probably better refine my mining and poking to make sure I'm not leaving any valuable loot behind!