BURROWING
Types: Innate, Tier 2.
Prerequisites: Strength, Claws.
Benefits: You gain a 10-foot burrow speed.
Special: This can be taken multiple times. Each time it's taken after the first it increases your burrow speed by 10 feet to a maximum equal to your base land speed.

TUNNELING
Types: Innate, Tier 3.
Prerequisites: Strength, Claws, Burrow speed.
Benefits: When you burrow, you can choose to leave behind tunnels equal to your size that last for up to 1 hour before they collapse unless stabilized with support per normal mining procedures.