The Underground Express was a method of transportation employed by the native Argonians in Black Marsh. It involved travelers submerging themselves into a Rootworm's stomach through their breathing holes. The travelers could then leave the Underground Express via the breathing holes as well. The travelers would then travel inside a Rootworm's stomach during their migrations. Notable exits and entrances to the Underground Express during the winter could be found at Gideon and Hixinoag.
Travelers could survive inside the Underground Express for several months without food, water, or air.