You can move through the earth by burrowing. Make a Creature Powers roll against the Toughness of the material; if you succeed, you can proceed at normal speed (not running speed) this action. If you fail, you are stuck and cannot go on. If you pick this ability twice, you create a safe tunnel that others may use. 

Typical materials and their Toughness ratings are wood (6), earth, plastic (9), stone, concrete (12), metal (15 or more).