in the selected block, there used to be a solid block (i can't remember what it was). poor jasper was the unfortunate duplicant assigned to dig up that floating block. when the block was dug up, he fell through the tile, through a block of granite and ended up where he is now, encased in a block of igneous rock.
alas, i couldn't get to him in time.
-floating block -duplicant assigned to dig up block -duplicant falls through tile, 2 blocks of air, and 2 blocks of solid ground ending up encased in rock.