Some species are able to tunnel through dirt and sand at ½ movement speed. Digging requires a full action to start the hole, once dug if your race is able to burrow. Burrowing through sand does not require an initial hole.
Cave ins can happen in two ways
Someone on the surface stands above the tunnel and succeeds an SR 15 vigor roll, +5 per 5 feet deep the tunnel is.
The character digging critically fails their roll to dig.
The digger must succeed an SR 15 Fortune roll or the cave begins to spread at a rate of 5 feet per turn in every direction.
Each turn the digger makes an additional SR 15 Fortune roll to see if the cave-in continues to spread.
The SR decreases by 2 for every 5ft away from the initial cave-in
If a character is caught in a cave-in, they begin suffocating. After 1 rotation they can begin digging at ½ speed if they succeed an SR 15 Vigor roll.