Until now, the analysis of burrowing mechanics has neglected the mechanical properties of impeding, muddy, cohesive sediments, which behave like elastic solids. Here we show that burrowers can progress through such sediments by using a mechanically efficient, previously unsuspected mechanism--crack propagation--in which an alternating 'anchor' system of burrowing serves as a wedge to extend the crack-shaped burrow. The force required to propagate cracks through sediment in this way is relatively small: we find that the force exerted by the annelid worm Nereis virens in making and moving into such a burrow amounts to less than one-tenth of the force it needs to use against rigid aquarium walls.