Burrowing
If you have a burrow speed, you can use move actions to tunnel through dirt. You cannot tunnel through rock unless you have an ability that states otherwise. You can move your full burrow speed while burrowing, but you cannot run (see page 248). Most burrowing creatures do not leave behind tunnels other creatures can use unless they have an ability that states otherwise; instead, the dirt closes up behind them as if they had not been there.
Source
Core Rulebook pg. 259
