There is a mod called floating ruins, It's in YogBox (try looking it up), and it creates floating dungeons with craters on the ground where they were ripped out of. Also, there are commonly rare ores on the bottom of the island. Inside, there is a monster spawner on top of a chest that's embedded in the floor. Sometimes there are even furnaces inside them.

There are three types of floating ruins, Snow Domes (spawn commonly in evergreen forests), brick buildings (spawn in regular forests and planes), and finally: Sandstone Pyramid (made of sandstone)