burrow speed

A creature that has a burrow speed, such as a purple worm or an umber hulk, can move through loose earth at the stated speed and through solid stone at half that speed. The creature can’t shift or charge while burrowing.

Published in Monster Manual 2, page(s) 216, Monster Manual, page(s) 280, Rules Compendium, page(s) 309, Monster Vault: Threats to the Nentir Vale, page(s) 122.