The Deep is a sandbox adventure for Worlds Without Number.
This adventure can be played as a standalone adventure for characters of about 3rd to 5th level, or as a follow-on from The Ruined Keep.
The adventure incorporates some wilderness exploration, a deserted village, and a three-level underground complex, 'the deep'.
Although this adventure was written for Worlds Without Number it is easily converted to any OSR roleplying game.