Fossil Cave, formerly known as the Green Waterhole and renamed on 4 May 1989, is a water-filled karst sinkhole in 30-million-year-old Oligocene coralline limestone beside the Princes Highway near Tantanoola. At the surface it is a modest depression about 20 x 10 m, but underwater it opens into a chamber roughly 70 m long and 30 m wide, reaching about 15 m deep. The CDAA describes it as a small pond leading to a broad chamber with dark silt at the bottom, and a small pool at the other end leading to a single small chamber.

Its fame is palaeontological. Since the mid-1960s divers and researchers have recovered Pleistocene subfossil material from depths up to 15 m, deposited mainly between 15,000 and 40,000 years ago. The species include the thylacine, the marsupial lion, giant kangaroos and two extinct birds – a giant coucal (Centropus colossus) and a large logrunner (Orthonyx hypsilophus). It is also the type locality for rare syncarid crustaceans otherwise known only from the Blue Lake. Fossils must of course be left undisturbed.

For divers the site is a Cave-category dive: Cave-rated divers may explore the chambers to 13 m but must not pass the signed restriction into the phreatic passage, which is for Advanced Cave divers only and forms part of the wider Tank Cave–Green Waterhole system (Wikipedia states Advanced Cave for all cave diving). The dark silt is the practical issue – it clouds instantly – so small teams and gentle technique are essential. It suits Cave-rated divers with an interest in natural history and good silt discipline.

Book through parks.sa.gov.au (listed under Piccaninnie Ponds Conservation Park) at least 48 hours ahead, with a signed indemnity per diver and CDAA verification. Drive about 22 km north-west of Mount Gambier on the Princes Highway towards Tantanoola; the site is adjacent to the highway (it is part of the Tank Cave/Green Waterhole system). Park as directed, carry gear to the small entry pond.

Three timeslots daily: 8-10 am, noon-2 pm and 4-6 pm, max five divers per slot.

Unaffected by weather; the constraint is the DEW booking (three two-hour slots daily, max five divers). Heavy rain can wash silt into the entry pond. Constant ~15°C water.

Highway noise at the surface. Slots are short (two hours), so arrive early to be kitted up at the start of your window; the site sits right beside the Princes Highway, so park well clear of traffic. Summer is no different underwater from winter, but hot days make kitting up in drysuits uncomfortable.

Enter via the small pond into the broad chamber, stay well above the dark silt, and circuit to the small pool at the other end that leads to a single small chamber, keeping within 13 m. Cave-rated divers stop at the signed restriction before the phreatic passage; Advanced Cave divers may continue on the line into the Green Waterhole/Tank connection.

CDAA grade: Cave (to 13 m, not beyond signed restriction) / Advanced Cave (phreatic passage) – 5L81, owner DEW. Book via parks.sa.gov.au ≥48 h; slots 08-10, 12-14, 16-18; max 5 divers per slot; min 2 per booking; max 2 slots/day; indemnity. Continuous guideline, thirds, three lights.

CDAA Cave and Advanced Cave members on DEW bookings; no commercial tours.