OK, so in the Beta, weirness is happening. First, a portal took me to the roof of the nether. What at first I thought were missing chunks turned out to be far above me. I was at height 46 or so, digging down I could go to height 1 everthing below that seemed to be void, no bedrock base layer. Returning to the roof, I saw something in the distance, turned out to be normal nether. Minutes afterentering this area, it disappeared, I was on bedrock roof. Returning to the original place, where I had entered a hole through bedrock into the nether this was now bedock. At this point I ave up, put everything important in an ender chest, found a misplaced chunk, and jumped into the void. Warning the nether is unstable at present!