A list of diggable terrain types:
indoors - sheltered city street - hard, diggable grassy field - diggable, plains light forest - wooded, diggable hills - diggable, plains mountainous - hard, climbable, diggable shallow water - swimmable, water_drinkable, diggable deep water - swimmable, water_drinkable, fishable swamp - poison_gas, diggable air - nothing desert - diggable paved road - hard, diggable underwater - swimmable, water_drinkable, underwater dusty road - diggable gravel road - diggable craggy mountain - hard, climbable, diggable, copper_mineable sheer mountain - hard, climbable, diggable, silver_mineable plain - diggable, plains snow - diggable, freezing, slippery ice - diggable, hard, slippery, freezing glacier - diggable, hard, slippery, freezing, salt_mineable beach - diggable stream - swimmable, water_drinkable, diggable, slippery, shallow, fishable cliff - climbable, copper_mineable chasm - climbable slippery floor - sheltered, slippery, hard, diggable mossy ground - diggable, slippery, plains dense forest - diggable, wooded foothills - climbable, hard, diggable, iron_mineable trail - diggable, plains jungle - diggable, wooded savannah - diggable, wooded rocky ground - hard, diggable void - nothing salt water - sailable dungeon - sheltered, hard, diggable lava - fire, diggable underwater-ground - diggable, swimmable, water_drinkable, underwater hay-ground - diggable, plains acid-pool - swimmable, acid mountain-road - hard, diggable bridge - hard, slippery fresh-water-sailable - sailable, water_drinkable, fishable fresh-water-swimmable - swimmable, diggable, water_drinkable, fishable, shallow ship-only salt water - sailable, ship_only