We've identified an issue in the current Minecraft Beta and Preview version that causes chunks to become corrupted below Y=0. To avoid corrupted chunks, please refrain from opening your worlds until an update has been released. - jhp

''We're open 7 days a week, 24 hours a day!'' HAH yeah sure, Mojang has one of the worst support systems. I sent a request 12/04 and has gotten 3 auto replys and the most recent one being in 18/04 by a real person, just to tell me i have to wait longer!!!