Can't see structures- looks like I have x-ray vision on but I don't.

A lot of blocks in parts of the world and also structures, hills and things I've built aren't there, although I can still walk right up to them. It is all black and looks like I have x-ray vision on but I don't. Also when I look at caves and hills I can see mobs and chests etc inside but I can't see the actual hill.