Tank Cave is the most extensive underwater cave in South Australia and the benchmark Advanced Cave site in the country. It lies near Tantanoola, is owned outright by the Cave Divers Association of Australia, and its entrance is underground: a short ladder leads down to a small surface lake, and from there the system spreads out into a maze of more than 7 km of diveable passage, around 10 km including the connected Green Waterhole (Fossil Cave) system. That makes it the second-longest underwater cave in Australia after the Nullarbor systems.

What makes Tank special is its character rather than its depth. Most of the cave lies shallower than 15 m, with typical depths around 10 m, so dives can last two hours or more with modest decompression. The passages are a labyrinth of low bedding planes, wider tunnels, chambers and connections, with white and brown limestone, fine clay banks and silt floors. Fixed lines run through most main passages, but side passages need jumps, and the sheer number of junctions is the challenge – navigation, line protocol and gas management matter more here than anywhere else in the region. Divers continue to find new passages and connections decades after exploration began.

Tank has seen deaths: two divers died in 2011, including explorer Agnes Milowka, and an experienced Victorian cave diver died in November 2025, after which the CDAA ran an internal investigation. Access is therefore controlled by an Access Officer roster and a staged familiarisation schedule. It suits CDAA Advanced Cave divers who want long, shallow, navigation-heavy cave dives – sidemount and rebreather divers especially.

From Mount Gambier take the Princes Highway north-west about 25 km towards Tantanoola; the property (owned by the CDAA) is off the highway and exact directions are issued to booked divers. Keep the property gate closed at all times. Park at the entrance area, carry gear to the underground entrance and climb down the short ladder to the small surface lake where divers kit up.

The site is open most weekends and by request; bookings through the CDAA website at least one week ahead, and a qualified Tank Cave Site Access Officer must be present each day.

Underground and unaffected by weather; heavy winter rain can raise water levels and cloud some sections. Access depends on an Access Officer being available, and farmland rules on Total Fire Ban days apply. Water is a constant ~15°C, so long dives need drysuits and heating.

Surface conditions only matter for kitting up at the entrance.

Follow the Tank Cave familiarisation schedule: early dives stay on the main lined circuits close to the entrance lake at around 8-12 m, using thirds on gas and noting every junction with personal markers. Later dives add jumps into side passages with a reel. Turn on thirds (or tighter for sidemount restrictions) and exit the same way; decompression is rarely significant at these depths.

CDAA grade: Advanced Cave only. Book through CDAA ≥1 week ahead; Site Access Officer required daily; follow the Tank Cave familiarisation schedule. Continuous guideline to the surface lake at all times; jumps need reels and personal line markers (cookies/arrows).

Rule of thirds minimum; redundant lights (3); sidemount advantageous in low sections; rebreathers common. Depths mostly <15 m – nitrox 32-36% ideal.

No commercial access – CDAA Advanced Cave members only, on days when a Tank Cave Site Access Officer is rostered (most weekends). Advanced Cave training and familiarisation dives run by CDAA instructors (e.g. Deep Blue Ventures, Diving Adelaide technical staff).