Divers map underwater caves that weave under city streets

An innovative project is bringing the hidden underwater world of South Australia's Limestone Coast to the public's attention. Divers are employing advanced photogrammetry and computer software to create detailed 3D maps of the region's complex cave systems, many of which lie directly beneath the streets and homes of Mount Gambier.

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With photography being an interest [of mine] … this idea came about using photogrammetry to try and map them.

Will Passos, an experienced cave diver and member of the Soggy Wombats dive team, is leading the effort. Inspired by his interest in photography, Passos uses a rig equipped with four action cameras and powerful lights. Each camera captures a photo every second, ensuring the accuracy and detail of the resulting 3D models. This method has been refined through trials, leading to successful models of sites like Pines and Iddlebiddy caves.

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There are old and good maps that people have been doing for the past 40 or 50 years, but it's been a while since anyone updated those maps.

The project aims to give people a better understanding of the subterranean landscape. Passos specifically wants to show residents what lies beneath their properties, highlighting the fascinating network of passages. By overlaying the 3D models onto street maps, the project visually demonstrates how the cave tunnels weave under Mount Gambier.

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I really want to be able to tell people what's under their houses and I think that's pretty cool.

This modern approach contrasts sharply with the historical methods of cave mapping in the region. Ian Lewis, a hydrogeologist and diver with decades of experience, recalls early mapping efforts as "primitive." These involved using measuring tapes, wrist-mounted compasses, and waterproof notepads underwater. The data collected was later used to draw maps on graph paper, similar to techniques used for dry caves. The new photogrammetry technique offers a significant leap forward in documenting these underwater wonders.

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We'd take [measuring] tapes underwater, we had underwater compasses on our wrist, we had little waterproof notes that you could write on with a pencil underwater.