Geologically, the Tasman Peninsula region is primarily mudstone/sandstone which in some areas has been extensively eroded to offer huge caves, tunnels and canyons, particularly in Waterfall Bay, an area well known for its sea caves.
The largest, Cathedral Cave, consists of a massive entrance (maximum water depth 21m), extending back into smaller caverns with narrow tunnels and cross passages, often with large schools of fish near the entrances. The walls are covered with brightly coloured invertebrates normally found in much deeper water. As you can imagine, this cave offers spectacular diving from a photographic aspect.
There are several other smaller cave complexes, including Dog Leg Cave and Horse-shoe Cave. The caves are for experienced divers with excellent buoyancy control, and diving in them is only possible when there is little swell.