The Undermarsh were a Nackers' network of muddy tunnels and caves which would, if it were truly part of the natural realm of the Marish, have been drowned in water. It was instead a special part of the Under-deeps, a netherworld of subterranean passages so-called because they underlay the sunlit surface of many lands in Middle-earth. The substance of the Undermarsh parttook of the realm of Shadows. The Nackers, and the more powerful spirits who had once created the Undermarsh, drew strength from Shadow and the dark essences of Arda. The Nackers worshiped these malevolent spirits, taking the magic that held their realm in existence to be a gift from He-Who-Walks-In-Darkness-And-Power, the Vala known as the Defiler, or as the Black Enemy.