I recently loaded up one of my really old beta worlds and every time I enter a region I previously visited (like, months ago) I end up in some weird nether world with no terrain, some surface cave backgrounds and no ability to jump. (And in one of the attached pictures you can see Ice Pirate shots frozen when they hit the bottom of the screen too.)
Basically, my question is this: is this endemic of the age of the world; is there a bug; or has my world simply become corrupted somehow?