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
One of our agents will reply to you as soon as possible. You'll receive the reply to the email address that was provided in the support request form. We're experiencing a higher than normal volume of requests right now, and we definitely appreciate your patience. ^FF
