My world was corrupted (only this world) when a "dark space" appeared, through which could be observed tunnels and lights on the other side of the space. Squid could also be seen (faintly) within the space that had been water. When I entered the space, I "fell", and could see my starting point well above (588 feet?). I used my pickaxe against the blocks forming the base of the visible terrain above to see if I could tunnel up, and "fell" again, this time about 7,300 feet (the void?). The starting point now appeared as a faint blue point, and I managed to fly in creative mode to the surface (14 minutes!) and return to the place I had left. Now, I cannot log back in, since the game stops at "building terrain".