Burrowing

is a innate mutation that and  have.

Burrowing
When the creature is bored and Burrowing is off cooldown, they will disappear into the ground and begin to move around. While burrowed:
 * Each tile that they pass over will spawn a wall based on what type of worm it is.
 * They get a Movement Speed penalty of 100 - 10 * MutationLevel.
 * They cannot attack or fire missile weapons without emerging from the ground.
 * They will be invisible until they are attacked, in which they are then forced above ground. Despite being invisible, running into a hostile burrowed creature will automatically attack them.