So just playing Minecraft on optifine and found a cave hole so thought, why not go down it? Turns out, I nearly die and find this infinite cave or ravine of some sort. It may be the texture pack was faulty, a strange seed or optifine broke. I'm not sure but there's a picture I've added so check it out for yourself.
Are you using mods? Either that, or it looks like something caused stone blocks to turn into air because I see what appear to be veins of ore floating in midair (this can also be caused by loading a modded world that replaces stone with a non-vanilla block in vanilla). Or this is some sort of x-ray mod or texture pack.