Forgotten Well is a dungeon found in the Shrouded Moors in Act II of Diablo III.
Clearing it can be nominated a bounty.
The well is a waterlogged cave, relatively small, but with lots of branches. It is filled with hordes of Skeletons and Skeleton Archers.
Its unique feature is that it's extremely dark, so that the character is barely able to see more than a few yards ahead. Daivin will refuse to come down if taken to the well, letting the Nephalem explore it alone.