Early into one game I found lava, and decided to try and redirect it in order to use it to power forges. Well, most of my dwarves had been working on the area, and the one redirecting the lava must have broke a hole in the pit and the level started flooding. I wasn't paying attention to them at this point as I was looking around on the service for other resources. When I checked back the entire room was flooded and only one dwarve remained, escaping to the surface. One of my lumberjacks went mad from grief, and killed the 2 other remaining dwarves.