The Cathedral is the second great feature of Piccaninnie Ponds – an enclosed underwater cavern with white limestone walls that looks like the nave of a church when the sun is high. National Parks SA and the CDAA both single it out: the parks agency describes it as a large underwater cavern with white limestone walls, and the CDAA calls it 'a beautiful white cathedral'. The overall depth of the chamber is about 35 m, with the most photographed part at around 10-12 m, where the bright limestone reflects daylight coming in from the pond and the Chasm.

Compared with the Chasm, which is about depth and exposure, the Cathedral is about form and light. The walls are smooth and sculpted by dissolution rather than collapse, the water is so clear that the chamber seems air-filled, and the few divers allowed in at once means it rarely feels crowded. Shrimp and small fish shelter around the entrance; deeper in there is little life. The floor is fine silt – one careless fin kick spoils the view for the rest of the slot, which is why buoyancy and frog-kicking are expected.

Piccaninnie's permit system historically let CDAA sinkhole-rated divers into the Cathedral and Chasm within set depth limits, and many divers booked two consecutive one-hour slots to see both properly. The site has been part of the ponds' long safety story, and the rules exist because of past deaths in the system. At present the whole park is closed to immersive activities after an algae outbreak and declining groundwater, with no reopening date. When it does reopen, the Cathedral will again be one of the most rewarding freshwater photography dives in the country for rated divers with disciplined buoyancy.

Drive about 32 km south-east of Mount Gambier via Wye to Piccaninnie Ponds Road and the Conservation Park car park, walk the short track with gear to the First Pond entry platform, then fin across the pond towards the Chasm; the Cathedral opens off the system close by. Entry only on a DEW permit, which is currently suspended; print permits in advance as mobile coverage is unreliable.

Weather has little effect underwater – there is no swell, current or tide – but surface conditions matter for the long gear carry and surface intervals. After very heavy winter rain surface run-off can cloud the upper few metres. Summer brings the best light shafts around midday and algae growth in the shallows.

Private and forestry land may be closed on declared Total Fire Ban days (common December-March), so check the CFS fire-ban forecast the night before. Groundwater levels in the Lower South East have been falling, which has caused closures of the DEW ponds sites in recent seasons. The Cathedral is closed with the rest of Piccaninnie Ponds while the ponds recover.

Cross the First Pond, locate the Cathedral entrance near the Chasm, and enter only as far as your grade and the permit allow, keeping the daylight zone in view. Hover mid-water to take in the white walls, avoid the floor, and exit the same way before finishing the dive along the Chasm wall and the pond weed beds.

CDAA grade: Basic Cave (sinkhole) on DEW permit; remain within daylight zone and permit depth; primary and backup lights; do not enter beyond grade – the rear of the chamber is an overhead.

Closed until further notice. Previously dived by CDAA-rated members on DEW permits and on trips run by Diving Adelaide and CDAA instructors.