The Chasm is the feature that made Piccaninnie Ponds famous. At the end of the shallow First Pond the weed meadow simply ends, and a slot eroded by the freshwater springs drops away into blue – the deepest parts of the spring system are recorded at more than 110 m, and in the region's exceptional visibility you can look straight down the walls far beyond any recreational depth. The upper walls are draped in green algae and aquatic weed; lower down the limestone becomes bare, grey and sculpted, and the light narrows to a band overhead. Descriptions from divers emphasise the sensation of hanging in air above an abyss, and Valerie Taylor famously rated it among the most beautiful sights in Australia.

That same clarity is the danger. There is no visual cue for depth, narcosis creeps up quickly, and in the late 1960s, 1970s and 1980s several divers died here. Those deaths, together with others at the Shaft and Kilsby's, drove the creation of the Cave Divers Association of Australia and the rule that only rated divers dive the Chasm under strict depth limits written into the permit. The operational depth limits were permit conditions and should not be confused with the geological depth.

A dive here is usually a wall dive: drop over the lip, choose one side, descend to your limit, look down, and come back up the other side. Freshwater fish, shrimp and eels are around the weed at the top. The Chasm links the First Pond with the rest of the system and is usually combined in the same permit slot with a look at the Cathedral. It suits CDAA-rated divers with excellent buoyancy and depth discipline – and at present nobody, because the ponds are closed while the ecosystem recovers from an algae outbreak and low spring flow.

Reached through the First Pond: drive about 32 km south-east of Mount Gambier via Wye to Piccaninnie Ponds Road and the signed Conservation Park car park, carry gear the short distance along the track to the entry platform and changing shelter, then fin a few minutes across the First Pond to where the bottom falls away. Access is by DEW permit only and permits are currently suspended.

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 Chasm is closed with the rest of Piccaninnie Ponds until National Parks SA lifts the immersive-activity ban.

Cross the First Pond, drop over the lip and descend slowly along one wall of the Chasm, holding a depth within your CDAA grade and permit limit (historically far shallower than the Chasm's true depth). Turn on gas or time, ascend along the opposite wall and finish with a safety stop in the weed at the lip before returning across the pond.

CDAA grade: Basic Cave/sinkhole rating required under the DEW permit; dive within the permit depth limit (historically 36-40 m range for Basic Cave). Deeper exploration is a research/technical exercise requiring trimix and special permission. Carry a primary light; no line required while in open water with daylight but maintain visual contact with the wall.

Closed to all diving and snorkelling until further notice. When open, dived by CDAA Basic Cave (sinkhole) rated members on DEW permits, including trips by Diving Adelaide and CDAA instructors.