Key points
- Students on a training voyage aboard CSIRO research vessel (RV) Investigator have helped shed new light on an intriguing shipwreck phenomenon.
- While involved in a shipwreck survey in waters off Bundaberg in Queensland, they observed mysterious holes in the seafloor surrounding a shipwreck turned popular dive site.
- We explain how these holes, likely caused by biological activity called bioturbation, can help pinpoint the location of hidden shipwrecks.
In September 2026, twenty Australian university students departed Brisbane for Fiji aboard CSIRO research vessel (RV) Investigator on a CAPSTAN sea training voyage. While in coastal waters off Queensland, the students were involved in several shipwreck surveys that led to an intriguing observation.
A shipwreck surrounded by mysterious holes on the seafloor.
For some onboard, this wasn’t the first time they’d seen such a thing.
The mystery of the circles
The CAPSTAN team were mapping the wreck of Althea II, a popular dive wreck lying in 40 metres of water off Bundaberg in Queensland, when they made the discovery.
Lara Picton, an honours student studying geology at the Queensland University of Technology, described what they saw.
"It was really exciting to watch as we mapped the site in detail. What initially looked like a few depressions in the surface turned out to be widely spanning," Ms Picton said.
"The entire wreck is surrounded by an almost perfect circle of holes in the seabed, each typically 8 metres in diameter and 2 metres deep. We identified around 300 holes and they extend approximately 100 metres from the wreck in all directions and are the only features on an otherwise flat seafloor."
Those aboard speculated that source of the holes might be biological. A drop camera was deployed from RV Investigator to investigate further and collect vision to test this theory.
The site was teeming with marine life of all shapes and sizes including large fish such as rockcod. The vision collected pointed to the holes being the result of bioturbation.
Bioturbation explained
Maddison Cross, a PhD candidate studying marine biology at the University of the Sunshine Coast, explained how marine life can reshape the seafloor.
“Fish, crustaceans and other marine life disturb the seafloor sediment as they feed, crawling, excavating and finding things hidden in the seafloor and may also dig burrows to lay protected within,” Ms Cross said.
“The process of livings organisms moving and mixing sediment is called ‘bioturbation’. The organisms responsible act as ecosystem engineers and can dramatically alter the shape of the seafloor,” she said.
So why are the holes so similar in shape and size?
Ms Cross explained that it might be the result of the daily goings-on of the life within the holes.
“We’ve seen Goldspotted Rockcod repeatedly using some of these holes so the similar shape and size of the holes might be because they reach a natural, ideal size for the life within them,” Ms Cross said.
The team onboard suggest the chief architects of the holes might be large predatory fish like the Goldspotted Rockcod who use the holes for shelter and ambushing other fish attracted to the wreck.
Intriguingly, a zone also exists immediately around the wreck where no holes are found. Potentially, this might be because the predators that live on the wreck make this a dangerous place to sit on the seafloor!
How holes helped solve a 74-year mystery
For CSIRO's Dr Ben Arthur, Voyage Manager on the CAPSTAN voyage, the holes looked very familiar.
Dr Arthur was a participant on a 2017 transit from Sydney to Broome. The voyage included a search for the wreck of SS Macumba, a 2,500-tonne merchant navy vessel sunk by Japanese bombers off the Northern Territory coast during World War II.
Dr Arthur said half a day was allocated to search for the wreck. However, as the hours passed, numbers dwindled in the Operations Room and those remaining began to lose hope the wreck would be found.
"It was after midnight, and just as the search was preparing to wind up, that an experienced biologist in the room noted a series of mysterious ‘pock marks’ on the seafloor at the very edge of the search area," Dr Arthur said.
"They suggested that these pock marks - circular holes likely caused by bioturbation - could point to the presence of a feature nearby on the seafloor."
A feature such as a shipwreck.
On the next pass, the CSIRO acoustic technician managing the survey asked the ship’s crew to swing the ship wide on the turn to take them outside the search area near the pock marks. Incredibly, this took RV Investigator right over the top of the wreck of SS Macumba!
"The jubilation of those in the Operations Room was a mix of excitement, exhaustion and incredible relief at finally solving this 74-year mystery," Dr Arthur said.
Without the telling pock marks on the seafloor, recognised by the experienced biologist and acted on by the astute acoustic technician and crew aboard RV Investigator, the wreck of SS Macumba might not have been found.
The science of shipwreck ecology
Bioturbation is not the only signal from the biological world that something might be hidden in the deep. Shipwrecks can create an oasis of habitat on an otherwise barren seafloor and produce a biodiversity hotspot.
Scientists call this 'shipwreck ecology.'
"Wrecks can result in greater concentrations of marine life when compared with the surrounding seafloor due to the structure and habitat they offer," Dr Arthur explained.
"They provide shelter and homes for crustaceans and small fish, which in turn attracts bigger fish. As a result, wreck sites are sometimes found by fisherman who are drawn to the increased numbers of fish around them."
In fact, tapping into this knowledge has helped lead RV Investigator and collaborative teams of researchers to identify several significant shipwrecks, including MV Blythe Star in Tasmania and MV Noongah in NSW.
The success of these searches relied on a combination of shared local knowledge, collaboration between teams of experts, and applying the impressive scientific capabilities of RV Investigator.
Ms Picton said the experience of bringing everyone together on RV Investigator, for research, education and training, was invaluable in helping unravel the mysteries of our oceans.
"Before I stepped onboard RV Investigator, I'd never had the opportunity to investigate bioturbation in this context or considered that such a thing could help in the search for shipwrecks," Ms Picton said.
"In the context of searching for shipwrecks, this phenomenon is super cool as it turns a small, hard to find footprint of a shipwreck in a vast ocean, into one spanning hundreds of metres."
Ms Cross highlighted that there are still more mysteries to solve about the marine life behind this phenomenon.
"It makes you wonder what more there is to discover about these unusual holes in the sediment and what calls them home."