Leviathan does not walk on the surface. It moves beneath it. The machine is a subterranean borer of staggering proportions — estimated at over 150 meters long and 20 meters in diameter, it tunnels through the earth at depths ranging from 30 to 200 meters. Its passage is detected by the surface effects: a rolling wave of displaced earth, collapsed terrain, and the deep subsonic rumbling that gives nearby settlements minutes of warning before the ground begins to buckle. Leviathan has been responsible for more infrastructure damage than any other Behemoth.

The tunnels Leviathan leaves behind are structurally sound — reinforced by a secreted mineral compound that hardens into something resembling concrete. The tunnel network beneath Iowa is extensive, and growing. Some sections have been explored and mapped; they connect Behemoth gathering points, mineral deposits, and underground water sources in a pattern that suggests deliberate infrastructure. Palisade builds roads on the surface. Leviathan builds them underground. The two networks intersect at specific points where vertical shafts connect tunnel to road, though no one has observed either machine acknowledging the other.

Leviathan surfaces rarely and unpredictably. When it does, the event is catastrophic — a section of ground the size of a football field erupts upward as the machine's boring head breaches the surface, venting heat and pulverized rock in a geyser of debris. It remains partially surfaced for hours, its upper hull exposed to the sky, before submerging again. During these surfacing events, the exposed hull radiates enough heat to be visible on infrared satellites. No one knows why it surfaces. No one wants to be nearby when it does.