"Ride the roots. The underground express."
―Chaero Gemullus[src]

The Underground Express was a method of transportation employed by the native Argonians in Black Marsh.[1] It involved travelers submerging themselves into a Rootworm's stomach through their breathing holes.[1] The travelers could then leave the Underground Express via the breathing holes as well.[1] The travelers would then travel inside a Rootworm's stomach during their migrations.[1] Notable exits and entrances to the Underground Express during the winter could be found at Gideon and Hixinoag.[1]

Travelers could survive inside the Underground Express for several months without food, water, or air.[1]