A creature that has tunneling leaves tunnels behind it as it burrows. The creature, as well as smaller creatures, can move through these tunnels without any reduction in speed. Creatures of the same size as the tunneling creature must squeeze through these tunnels, and larger creatures cannot move through them at all. See also burrow speed.
Published in Monster Manual 2, page(s) 219, Rules Compendium, page(s) 317, Monster Vault: Threats to the Nentir Vale, page(s) 125.