So when your party enters a dungeon you think it all looks like the following image? All nice and dirty with lots of darkness ahead?
Well got news for you. Underground passages are damp and humid. Water drips from the roof and pools on the floor. As it collects it flows to the lower parts and builds up there. Water alongside gas have been an everlasting concern in mines, what would make us think it is different in dungeons? A rainy season can bring flooding to an otherwise empty dungeon. Turning it into something like this:
Flooding alongside rock hardness were the limiting factors for serious mining in the early middle ages. Only when the issue of flooding was solved with water extraction mechanism was deep earth mining possible. Such systems like the one shown below allowed for the pumping of water from the deep tunnels out to the surface.
A building like the one below could be a tell tale sign of a dungeon existing below its foundations.
The need for water pumps (mechanical or magical) can also be a great source for adventure. Maybe the pump is in a remote part of a mine and the party is asked to help fix it. A team of "engineers" are willing to go fix it, but the party is needed to flush out the monsters and clear the way through the deeper parts of the mine which are now abandoned.
Maybe the pump is of magical nature and the party can disable it to flood the dungeon and thus entrap a terrible threat about to break out. Or the first part of the adventure is rehabilitating the pump so it clears the water out from the deeper sections of the dungeon. This will allow the party to continue into the deeper areas of the maze.