This refers to levels 9-12 of Diablo I. For the Diablo II location, see Caves (Diablo II).
The Caves are the third set of levels in Diablo I, consisting of levels 9-12. It appears more like the Christian idea of hell than the actual Hell levels do, with large lakes of lava and stone outcroppings.
Staircases appear like
There are no shrines, but barrels do appear. Walls and doors are replaced by fences and fence gates, but some monsters can open the gates.
Just like how every first level of a level set has a staircase up to town, so do the Caves; the entrance is a hole in the cliffs near Wirt. Mousing over it says "DOWN TO CAVES."
Very few quests, and none in multiplayer, take place in the caves: