This apparantly does nothing then disappears, which is a shame becuase it was pretty. Im not sure if its a bug or some obscure event but it should probably be less obscure if it is.

Stuff like this should not disappear as you walk on it, empty tiles are not really used for anything anyway. If an outpost, city or new quest is created on top of it then fine, but otherwise it would be nice to just leave ruined fortresses, eldritch pits and giant trees where they are. The lack of icon should be enough to let us know that there is no treasure inside and scattered ruins would pretty up the blah map.