When they were up in the air above it, they could see that in middle of the valley, there was something peculiar - a little river was falling down into a big hole in the ground. However, it must had been falling so far below, that no sound of river splashing the ground could be heard - it was apparently a peculiar, deep, bottomless pit.
https://s-media-cache-ak0.pinimg.com/564x/a0/00/4a/a0004a59dca6c493d5ee6f57112a4a3a.jpgNow, humans on their own would probably have little to no chance reaching the bottom alive... But someone with ability to fly down, well, that was already another story....