The Pit is the "cave" in the Tamoe Highland. The Tamoe Highland's can be found in Act 1, between Black Marsh and the Outer Cloister (just outside the Monastery). Just follow the path. There'll be a fork at some point, and one way leads to the Pit.
People like to do magic find runs there on Hell difficulty, because it can drop all the items in the game (though there's a smaller chance). It's also pretty good experience.
I alternate between Pit and Cow runs with really low MF because I'm searching for socketables and each gives the best odds for certain specific highly desirable ones. Even with just a P-Topazed Shako, I still pick up ~ 1 unique every time I run both.