The Dead Marshes are actually just outside of North-Western of the Black Gate of Mordor. These marshes was actually an ancient battlefield of the Battle of Dagorlad between the Last Alliance and the forces of Mordor. As time passed, the battlefield became turned to the marsh land that it is now. The dead were swallowed up by the dark waters, though their corpses can still be seen floating in the water. An ancient graveyard.
