Finding some weird floating blocks in a grid pattern. They aren't solid so I can walk through them without suffocating. Sometimes you can mine the blocks on it and there won't be particles or sounds. If you place a block on it, all the blocks will turn into the block you just placed. I've only seen these in survival mode.

One of these screenshots was taken a few months ago in a beta version for 1.14.0

(Sorry if my grammar is bad)