On an ice world where acid runoff steams through the trench lines, a human colony is trying to evacuate eight thousand civilians from a failing hab-dome to a granite mesa. Beneath the frozen shale live the Burrow Tusks: armored subterranean beasts that hunt vibration, crack open exosuits and vehicles, and use the warmth inside human armor, engines, and supply compartments to nest their young. One last engineer is sent into the western trench to plant six thermal charges and collapse the warren long enough for the civilians to escape.

Inside a failing Mark-IV sapper frame, he moves through rising acid water, hollow duckboards, damp fuses, and ground that answers every step with a dead note. The mission begins as a demolition job: plant the charges, sync them to the seismic hammer, break enough tunnels to buy two hours. But the Tusks are not  ...