Little Blue Lake: the South Australian sinkhole with a Morris 1100 on the floor

There is a 1966 Morris 1100 sitting on the bottom of Little Blue Lake. Also an old petrol bowser, some traffic signs and a scatter of witches hats. None of it was put there for divers. It was dumped over the years into a hole in the limestone, and the water simply kept it.

That is the strange charm of this place. It is a cenote in farm country, about 20 kilometres south of Mount Gambier in South Australia, in the locality of Mount Schank. Locals swim in it. Divers come from across the country for it.

If you like freshwater, geology and water that does not behave like the ocean, Little Blue Lake is worth the drive on its own.

What the site actually looks like

From the rim it is a circle of water roughly 50 metres across, ringed by vertical and undercut cliffs about 8 metres high. You get to the water through an artificial cutting on the southern side, down a metal stairway to a small floating pontoon. The District Council of Grant put the stairs and pontoon in back in 2002. Parking is on the east side.

Underwater the floor is very flat and silty, averaging around 36 metres. The highest point of the bottom, about 25 metres, is the top of a rubble pile directly beneath the cut access ramp and pontoon. Around the southern and western sides the floor darkens and gets silt-prone, running to a maximum recorded depth of just over 42 metres.

Visibility here is normally poor, though it can improve at depth. The water table moves too: seasonal and longer-term shifts can raise or lower the lake by around 4 metres.

Why it is called blue when it is green

The name borrows from Mount Gambier's much larger volcanic Blue Lake. Little Blue Lake used to turn blue in summer the same way. These days it generally stays green most of the year, which some attribute to groundwater picking up nutrients from agricultural fertilizers. You may also hear it called Baby Blue, or see it written as 5L9 in caving literature.

A hole made by a volcano, sort of

Little Blue Lake is one of a cluster of similar landforms on the flat coastal plain south of Mount Gambier and around the dormant volcano at Mount Schank. They share a look: circular collapse dolines with cliffs, lakes filled to the water table, and big rubble cones on the floor. The rock is Gambier Limestone, Miocene in age.

What makes them different from other karst in the region is their relative depth, their water chemistry, and the absence of underwater phreatic passages. One nearby cenote reaches 125 metres. So these are not the flooded cave systems you might picture.

The theory in the Wikipedia article on Little Blue Lake is that large water-filled chambers collapsed after sea levels dropped at the last Glacial Maximum, about 20,000 years ago. The chambers themselves were likely dissolved by groundwater acidified by carbon dioxide rising through fractures from magma chambers, rather than by the usual atmospheric route.

From around 8,000 years ago, as sea levels rose again, they filled with freshwater. Stromatolites are present in at least eight of these cenotes, Little Blue Lake included, and that is taken as a sign of how recently they formed. Living rock, in a hole made by a volcano's breath.

Who has been down there before you

Divers started exploring the lake's underwater environment in the 1950s. Lewis and Stace surveyed the submerged extent in 1980, and the Cave Divers Association of Australia surveyed it again during the 1990s. The land itself was dedicated as a reserve "for the purposes of a Public Pleasure Resort" on 19 March 1986.

Weather on the coast

Those numbers are marine figures for the coast nearby, not for the sinkhole, which sits inland of the water. Still, they tell you what the weather is doing on your trip. Winter swells out there run above four metres in July and September, and the calmest stretch on paper is high summer in January.

Getting there

The lake is on the north side of Mount Salt Road, about 3.5 kilometres west of the Riddoch Highway. That highway is the main run between Mount Gambier and the coastal fishing township of Port MacDonnell, so it is an easy detour if you are working your way down to the coast. Mount Gambier is 20 kilometres away and makes the obvious base.

Pair it with the other cenotes in the district and you have a trip built entirely on holes in the limestone. That is a very particular kind of weekend, and some divers never want to dive anything else afterward.

If you make it out to Mount Schank, log the dive in Scubaspot with the depth you reached on that flat silty floor, and note what the visibility was doing that day. A photo of the Morris 1100 would be a gift to everyone planning the same drive.