Leviathans

The leviathan is probably one of the worst nightmares living on Mars.

Description
This huge worm waits patiently beneath the surface until it detects nearby prey, sensing its vibrations through the ground. It then shoots violently out of the ground to attack with its many tentacles, before dragging the prey to its teeth-filled maw, in order to eat it. The only chance to win against this massive monster is to destroy the very tough, armoured maw, as severing single tentacles will simply cause them to re-grow.

Only well-equipped adventurer parties can survive an encounter with one of these horrors. Even worse, the leviathan can appear anywhere on the surface of Mars.