Brilliant, Holmes!
Working our way through the dungeon, the corridor we are in opens out in to a larger area. The DM draws on our gaming mat what we see, telling us that the bit in a middle is a river, and the cavern continues on the other side about as far as it does on the side we are standing.
The Paladin player, who must have sneakily bought some Eyes of the Eagle, says, 'So it's a cavern with a river running through it.'
The DM awarded him an experience point for such amazing powers of observation.