You wouldn't know it today, but a very long time ago – around 100 million years – Australia was a continent divided by water. What is now central Australia, the flat, infinite radial plain that lies between Adelaide and Darwin, was once the floor of shallow sea. This is also how South Australia came to have caves full of crystal clear water in the middle of the desert.
Most caves elsewhere in the world were formed when streams carved holes into ancient fault lines, but South Australia was a little weirder. The middle Australian state is bookended by two huge sheets of limestone that formed under the sea. Then later, when the sea drained away, rain gnawed into the limestone to form sprawling networks of water-logged tunnels that disappear for kilometres into the guts of the earth.
None of this was much understood until the 1960s. That's when an Adelaide-based hydrologist named Ian Lewis, among others, started exploring these outback caves and realised just how deep they go. Today Lewis is the public face of the Cave Divers Association of Australia but when he started diving, the sport didn't really exist.
People knew there were deep caves in Mt Gambier and the Nullarbor but that was about all. Even now he remembers that initial sense of wonder when he lead a convoy of divers out to the desert in a green Holden Kingswood during the 70s on an expedition to map previously unknown sites.
"It was something no one had done before," he said. "And the diving was breathtaking. The caves were enormous. The walls were pure white, so our lights reflected off everything. The water was crystal clear and pure. When we surveyed them, the divers were 500 feet apart and you could see them as crystal clear as day."
Since then, times have changed. Now there are new faces and new technologies like rebreathers, which extend the amount of time a diver can spend underwater. Lewis himself doesn't do much exploring these days. Instead he leaves much of it to young guns like Dr Richard Harris.
Harris is an anaesthetist by trade and a member of an elite group of cave divers called the Wet Mules, named for their willingness to voluntarily haul heavy equipment to remote caves and spend up to 20 hours following them from start to finish. Harris, or "Harry" as he likes to be called, described diving into a cave like "swimming through air".
"Some people look down through the clear water and feel dizzy," he said. "There's a well-known site in Piccaninnie Ponds, where there is a narrow chasm or slot that drops down about 20 metres and as you swim out over it, the ground drops away out from below you and a lot of people feel like they're going to fall."
"Some people overuse the space analogy, but it's really appropriate. You really do feel like you're suspended in nothing."
Cave diving at an elite level is about mastery and precision. The level of technical knowledge needed to pull it off is one of the things that separates it from other exploration-based activities like Urbex. Those who make it that far in the cave diving world mark themselves out as explorers and careful, "technical-minded" divers chasing the perfect dive. A clumsy dive that stirs up silt or disturbs the cave is a professional sin.
To even get to that point takes months of dedicated training, practice, and experience, and access to cave sites is difficult. This is because hazards are real, even if the community are sensitive to public perception and would prefer to not talk about them. When a person dives into the ocean, they have light and can just resurface if they get into trouble. A cave diver cannot. "Thrill-seekers" and "suicide freaks" are not welcome.
"If you want that, you may as well take up skydiving," Lewis said. "We go in wanting to come out again."
While it's all he would say on the subject, things do go wrong and divers do fall in the line of duty. Between 2010 and 2011 the community lost three members, among them marine archaeologist, Agnes Milowka, a highly respected diver from Victoria who died in 2011 while exploring Tank Cave, one of largest and most complex cave systems in the world.
Harris was on the dive team that helped police recover Milowka's body. Milowka, an experienced diver, had become separated from her diving partner while exploring a tight nook. In doing so, she kicked up a cloud of silt, became disorientated and couldn't find her way out.
Though he didn't know her as well as some, Harris knew Milowka and the pair had worked together on the set of Sanctum.
"She was a very happy, gregarious person, full of life, full of adventure. A very high level cave diver, extraordinarily talented, but a little too bold as it turned out, as that's what got her into trouble," he said. "She was a real explorer."
Probably the most lethal period in cave diving history came in the 70s when 11 people died in the space of six years. This is what prompted the creation of the Cave Divers Association and the introduction of a training program designed to save people from themselves. Since then, Australian cave diving has a fairly good safety record and until 2010, Harris said there hadn't been a single fatality since 1985.
And while good portions of about 50 known cave sites across the state have been mapped, there is still more out there. Divers call finding something new "laying line" and every now and then someone comes back having mapped a new stretch of tunnel.
This has also tended to put the diver on the front-line of scientific research as they return with water and rock samples, fossils, or discover strange new forms of life. As for undiscovered caves, Lewis says they're out there, but the challenge is finding a way in.
"The limestone is full of holes, you just have to find them," Lewis said. "There's a chance of finding new caves, and the technology to find them is getting better and better. There are 12,000 blowholes on the Nullarbor Plain. Air belts out of them or sucks in. Some may have a cave at the bottom, some may not. No one knows."
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