Thunderhead is the tallest confirmed Behemoth, a spindly tripod structure that rises nearly 30 stories into the sky. Its three legs are impossibly thin for its height — each one a lattice of carbon-fiber and unknown composites that flex visibly in high winds but never break. At the apex, a bulbous sensor dome bristles with antennae, radar dishes, and atmospheric sampling equipment that gives the machine the appearance of a walking weather station. Which, in a sense, it is.

Thunderhead generates its own weather. The electromagnetic systems in its dome interact with atmospheric conditions to produce localized storm cells — clouds form around its upper structures, lightning arcs between its antennae, and rain falls in its immediate vicinity regardless of regional weather patterns. The storms intensify when the machine is stationary and dissipate when it moves. Meteorologists who have studied the phenomenon believe Thunderhead is actively manipulating ionospheric charge layers, though the purpose is debated. Some believe it is harvesting atmospheric electricity for power. Others think the weather modification is a side effect of whatever it's actually doing up there.

The machine moves in long, deliberate strides across the prairie, each step covering fifty meters. Its passage leaves behind soil that is measurably more fertile than the surrounding wasteland — the combination of electrical discharge and rainfall appears to revitalize dead earth. Wasteland farming communities have learned to follow in Thunderhead's wake, planting crops in the rejuvenated soil. This has led to a strange symbiosis: human settlements that migrate with a machine that doesn't know or care they exist.