Tunnel worms live in mazelike complexes of burrows connected by passages they have chewed out over time; these burrows are often filled with the rotting remains of past prey, used by the worm to incubate its offspring. The favored tactic of the tunnel worm is to lurk beneath the surface of the ground or behind a wall until it senses a creature moving by, whereupon it burrows out to surprise and attack its prey (treat this attack as a charge attack). A tunnel worm that has taken more than half its hit points in damage retreats to its lair unless it is cornered, in which case it fights to the death.