The subject can burrow at a speed of 15 feet (or 10 feet if it wears medium or heavy armor, or if it carries a medium or heavy load) through sand, loose soil, or gravel, or at a speed of 5 feet through stone. Using burrow requires only as much concentration as walking, so the subject can attack or cast spells normally. The burrowing creature cannot charge or run. Loose material collapses behind the target 1 round after it leaves the area. This spell does not give the target the ability to breathe underground, so when passing through loose material, the creature must hold its breath and take only short trips, or else it may suffocate.
The target can breathe underground while burrowing. If you're 3rd tier, the burrow speed increases to 20 feet through sand, soil, or gravel, and to 10 feet through stone. At 6th tier, these speeds increase to 30 feet and 15 feet respectively. At 9th tier, these speeds increase to 40 feet and 20 feet.
Augmented (3rd): If you expend two uses of mythic power, the burrowing creature may leave behind an open tunnel with a diameter equal to half its space.
When the spell's effect ends, this tunnel closes up immediately.