Extract from ABC News
The mapping is highly detailed, with the deepest parts shown in purple. (Supplied: CSIRO)
In short:
Researchers have confirmed Lake St Clair's true depth at 163 metres at its deepest point after completing the first high-resolution 3D-map.
The project used advanced multi-beam sonar and LIDAR technology, which revealed dramatic underwater features.
What's next?
CSIRO will share its findings to improve safe navigation and guide future scientific research.
Researchers from Australia's national science agency have measured Tasmania's Lake St Clair with high-resolution detail, creating a 3D-map modelled from sonar pulses.
Past estimates of the lake's depth, provided in the 1860s and 1965, had put it at between 160 to 215 metres.
But the recent CSIRO research definitively places Lake St Clair at 163 metres deep, nearly twice the depth of Bass Strait.
There have been two previous attempts at surveying Lake St Clair. However, both produced inaccurate readings. (Supplied: CSIRO)
CSIRO hydrographic surveyor Augustin Déplante said the finding has put much speculation about the lake's size to rest.
"Our mapping confirms that Lake St Clair is absolutely Australia's deepest lake, with the next deepest lake being less than 100 metres deep,"he said.
Mr Déplante said he "felt quite compelled" to map it himself after discovering that Australia's deepest lake had never been measured using modern technology.
Lake St Clair lies at the southern end of the Cradle Mountain–Lake St Clair National Park within the UNESCO-listed Tasmanian Wilderness World Heritage Area.
Its 15-kilometre length marks the finishing point of the iconic Overland Track, a multi-day bushwalk.
Augustin Déplante says the presence of a Lake St Clair 'Loch Ness' monster was not recorded. (Supplied: CSIRO)
Mr Déplante said the mapping was so detailed that objects as small as 50 centimetres across were identified.
"Along the shoreline, it shows trees that have fallen into the lake, and in deeper areas has revealed several mysterious features on the lake bed, sparking curiosity about their origins," he said.
"While the data does not confirm the presence of a Lake St Clair 'Loch Ness' monster, it does offer a powerful new tool for exploring the lake's hidden depths."
Dramatic underwater cliffs, deep ravines
A hydrographic survey team spent eight days battling wild weather to systematically map the freshwater lake.
Mr Déplante was assisted by two other surveyors, a skipper and an engineer.
The team used two multi-beam sonars, which measure water depth with sound pulses, and Light Detection and Ranging (LIDAR) technologies.
The high-resolution 3D map identified fallen trees in the lake. (Supplied: CSIRO)
The results were integrated into a single 3D dataset, which provided intricate details about the lake bed and shoreline, including dramatic underwater cliffs, deep ravines and towering rock formations.
A remotely operated two-metre vessel, known as the Otter, enabled mapping in shallow areas.
Otter, a 2-metre-long remotely operated vessel enabled mapping in shallow areas inaccessible to larger vessels. (Supplied: CSIRO)
Previous attempts at surveying
The earliest attempt at surveying Lake St Clair in full took place in the 1860s using lead lines, a method now considered outdated.
It involved lowering a line with a lead weight until it touched the lake bed, then recording the length of the line to estimate the depth.
This attempt provided a depth estimate of 168m.
A twin-hulled 8-metre research vessel, RV South Cape, and a 2-metre unmanned, remotely operated vessel called the Otter. (Supplied: CSIRO)
Mr Déplante said many factors may have contributed to this overestimation.
"The lead could have gone into the mud without them realising and [the line lowered] deeper," he said.
"It's also possible that at this depth, the line could be dragged a little bit by the current, making the line longer [and adding depth to the measurement]."
The most recent attempt was in 1965 using a single beam sonar, which also had limitations.
It could not compensate for variations in the speed of sound throughout the water column and the sonar was not able to pulse fast enough to capture accurate data.
This attempt provided a similar depth estimate to the previous figure.
CSIRO plans to share the 3D mapping with others to support increased understanding of the lake's underwater habitats and origins, assist safe navigation on the lake, and inform future research.
"It will definitely be useful to the scientific community and benefit the habitat as well,"he said.
Mr Déplante hopes the research method will be replicated on other waterways around the state.
"When you look at the River Derwent with the spotted handfish, they are a pretty rare species in Tasmania, maybe it could help with that as well."
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