Graphics glitch

When playing minecraft (whether in creative, adventure, or survival mode), there sometimes is a glitch in the graphics, causing a large area of blocks to be see-through; so instead of seeing the blocks themselves, I can see "miles" underneath.

I've also noted several "missing texture" icons on some of the new blocks when destroyed.

Lastly, as I was playing a recently created level, and was digging my way up, I encountered what appears to be a missing chunk. A very, very large square of missing blocks, going all the way up and all the way down, with everything visible all around. I didn't dare step into it; I didn't want to die. I've enclosed pictures of two DIFFERENT missing chunk holes.

And then... it's the end of the world as we know it???