FOSS gamedev and creative worlds

Hi everyone. I'm new at Minetest, and I was just wondering about some weird structures I came across in the game. I understand the worlds are randomly generated, but in one of my maps, I've been coming across structures that look intentionally built, although I know the map was created on my computer. The first thing I noticed was a stone rectangle that's floating in midair close to the original spawn point of the map. Also, there's a large, squared-off room in a deep cave that has little halls and stairs that lead to smaller square rooms. I've come across these rooms and stairs in 3 or 4 places on the map, and they're always made of mossy cobblestone, for some reason. Does anyone have an explanation for these features? I'm using Minetest 0.4.13. This particular map was created on a Windows 7 computer. I have attached a couple of pics (I put the torches in the cave myself).

The squared off room is a dungeon. It's supposed to look man-made. You can turn dungeon generation on or off in your settings. The floating stone is probably also a piece of a dungeon (though obviously it's not supposed to generate like that).

Ok, I've heard of the dungeons, but I didn't know that's what I found. I like them. They remind me of Mayan ruins or something like that.

Yeah the floating rectangles are unintentional slices of dungeons, those are now removed in 0.4.14.