Rootworms were creatures that were native to the swamps of Black Marsh, the homeland of the Argonians. They were massive worms that rode along the roots of the trees under the swampy water of Black Marsh. They had thin skin as well as breathing holes that travelers could make use of to enter rootworms' stomachs and ride inside them. This method of travel was referred to as the Underground Express. They were approximately eight feet long and were colorless. They also did not possess eyes or bones.
During the winter in Black Marsh, the rootworms migrated from northern regions and towns such as Hixinoag to southern regions and cities such as Gideon. They were also extremely fast and could cover the distance between Hixinoag and Gideon in several minutes.