Blue Holes are underwater sinkholes, and are noted for the stark contrast between their light blue surroundings and the deep blue color of the sinkhole itself. Circulation within the hole tends to be poor, and the water is often anoxic. They formed during the last ice age when the ocean level was lower and erosion formed those holes within the underlying limestone. They typically have a diameter of 25-300 meters, and a depth of 100-120 meters, although the deepest Blue Hole in the world is 202 meters deep.
- Lusca - a monster said to dwell in such holes
Game and Story Use
- A circle may serve as a gateway from another world, as in fairy circles. And the aspect of depth to blue holes hints that creatures from the stygian underworld might emerge from it.