Originally posted by ShaftAlmighty:

Just further confirming, i've found those spiral staircases in both the swamps(submerged) and the mountains. There is nothing to them, not even a chest. Perhaps in a future update there may be an accessible dungeon or some loot, but for now it's just pointless detail landmark.

The mountain ones can contain a chest but it's random and not a sure thing. The mountain ones can be turned into a pretty cool base from the screenshots people have posted.