Ruin

From DwarfFortressWiki

A ruin is a former human or dwarven settlement that is now filled with undead. These sites are discovered in adventurer mode, either by stumbling across them or by talking with townsfolk and being alerted to nearby sites. These will then show up in fortress mode when picking a starting site, and the player can choose to place a new fort on top of a ruin or cave. Beware, your starting dwarves will probably be massacred!

Overworld
World generation - Regions - Biomes - Civilizations - Map legend - Sites - Armies
Aquifer - Brook - Cave - Chasm - Forest - Lake - Mountain - Ocean - Pit - River - Volcano - Waterfall