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The mystery of Australia’s deepest lake has finally been solved
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The mystery of Australia’s deepest lake has finally been solved

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Lake St Clair is deeper than Bass Strait and is Australia’s deepest lake, but its underwater landscape has been largely a mystery until now.
For years, people have speculated that the lake – which marks the end of the internationally renowned Overland Track, a multi-day walk in the Tasmanian Wilderness World Heritage area – could be as deep as 215 metres. But a CSIRO team thinks they have finally solved that mystery.
The team spent nine days mapping the lake with technology that, according to hydrographic surveyor Augustin Déplante, showed them “as if they had drained the lake and were walking around in it.”
“I really wanted to find out what the deepest point of the lake was,” Déplante said.
“There have been some wild numbers and I was never able to figure out what that referred to, whether it was just the depth of the lake on that day because it had rained a lot, or whether it was tied to an actual elevation level.”

Now he has an answer: the deepest point of the lake is just over 163 meters deep.

CSIRO maps reveal a stunning new view of the lake and its true depths. Source: Delivered / CSIRO

“It’s on the south west side, basically just in the corner,” he said.

“We can almost see that the glacier would have actually been cut through that angle.”

Underwater landscape revealed

Using advanced multibeam sonar and Light Detection and Ranging (LIDAR) technologies, the team created a 3D map of the lake bottom and shoreline.

A multi-beam was fitted to the CSIRO vessel RV South Cape as it sailed up and down the lake. This used sound pulses to measure the water depth, creating a never-before-seen view of the lake.

Augustin Deplante.jpg

CSIRO hydrographic surveyor Augustin Déplante was part of the nine-day expedition. Source: SBS news / Kerrin Thomas

“When we see the sharp cliffs at the edges of the lake, we can clearly see some areas that have been carved by the glacier,” Déplante said.

“You can see the deposits of the glacier where it would have stopped retreating and dropped a lot of rocks and sediments and so on, and then kept going.”
The underwater features are clearly visible on the 3D image.
“We can see the supply of fresh water coming from the mountains into the lake and it cutting through the bottom of the lake, so we have these waterfalls actually coming down into the lake and then reaching the cliffs, it’s quite incredible to watch.”
A multi-beam and LIDAR were mounted on a remotely piloted vessel called the Otter to survey the coastline.

“It can go very shallow in areas of less than two metres, even less than one metre, as long as there are no hidden rocks or anything below the surface,” said CSIRO research engineer Dr Hui Sheng Lim.

An Asian man wearing glasses with his arms crossed and smiling. He sits in front of computers showing maps and diagrams of a lake.

Dr. Hui Sheng Lim was one of the CSIRO researchers who mapped the lake. Source: SBS news / Kerrin Thomas

“So those are the areas where the big ship can’t go.”

The Otter revealed areas close to the coast with sharp escarpments.
“You’ll see how quickly that actually fell down from the shoreline, down to 50 meters,” he said.

“You see a meter here, and the next moment you’re in a 10 meter zone, and the next moment you’re already at 30 meters, 50 meters deep.”

A rocky coastline and lake. Clouds and snow on the hills can be seen in the distance.

The area is known for its strong winds and unpredictable weather. Source: SBS news / Kerrin Thomas

Otter can also operate autonomously and follow a precise, pre-planned path.

“One of the most useful features is that we can autonomously find gaps and find the best way to close those missing gaps in our data sets,” he said.

‘Most peaceful place on this planet’

Michael Croghan has been running the Lake St Clair Lodge on the south side of the lake for almost 20 years.
“Personally, I would describe it more as my church,” he said.
“It’s the most peaceful place on this planet. I may be biased, but so many other people say the same thing. It’s visually stunning, never the same two days in a row.

“It’s the most phenomenal place to swim because it has the cleanest air and water in the world.”

A middle-aged white man holding up a bottle of gin. Behind him is a bar with various alcoholic drinks.

One of Michael Croghan’s most prized possessions is a bottle of gin lowered to the bottom of the lake. Source: SBS news / Kerrin Thomas

One of his most prized possessions is a bottle of gin that a team of researchers lowered to the bottom of the lake a few years ago.

“Just for a giggle, I said to them one night, would you like to take a bottle of gin, put it at the bottom of the lake and take a picture? And they said, yeah, sure. So I thought, okay, great. So they did.”
The bottle is always at hand behind the bar.

“It has now been tested to depth, I think that was to about 400 feet,” he said.

Snow, wind and swell

Tasmania’s wilderness is known for its unpredictability and the team encountered snow, large waves and gale-force winds during their work.
“It was amazing: it’s so wild around the lake, you’re in the elements all day,” Déplante said.

“When I was on a small boat like the South Cape, I sometimes felt like I was in the Southern Ocean.”

A man, dressed in a high yellow fishing suit, on a boat in a lake

The team was confronted with snow, large waves and gale force winds during their research. Source: SBS news / Kerrin Thomas

The detailed data collected will be made public and will provide an important resource, which it is hoped will stimulate further study of Lake St Clair.

“When I saw that Australia’s deepest lake had never been explored with the multi-beam I was very surprised, but I think if it was easy to do it would already be done,” Déplante said.
And while the lake is known as a home to platypuses, it’s unknown if something bigger lurks beneath the surface.
“No weird monsters or anything yet.”

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