Few people on Earth have seen a polymetallic nodule 4 kilometres under the ocean, but Rob McCallum is one of them.
"They are anywhere between the size of a potato or a baseball, and they are black," he said.
Mr McCallum is a divemaster and founder of EYOS Expeditions, a firm that specialises in deep-sea diving operations.
He has explored the depths of all five of the world's oceans, including the Mariana Trench and has made trips to the wreck of the Titanic and the Bismarck.
He has also descended into the Clarion–Clipperton Zone — a vast area of the Pacific that's been identified for prospective deep-sea mining projects.
This area contains countless polymetallic nodules formed over millions of years.
They contain valuable critical minerals including nickel, cobalt, copper and manganese, used to create batteries for phones and cars.
Mr McCallum likens them to nuggets from Australia's gold-rush era.
He's not alone.
Billions have been invested in exploration and research into deep-sea mining, as governments and companies position themselves to tap into a new source of critical minerals.
United States President Donald Trump last year signed an executive order to accelerate seabed mining in US and international waters, arguing it could help secure supplies of critical minerals and reduce dependence on foreign suppliers — primarily China.
In August, the US went a step further by announcing plans to sell deep-sea mining leases in US-controlled waters around the Marianas and American Samoa.
Meanwhile, mining companies have forecast billions in potential profits, with royalties flowing to Pacific nations willing to open their waters to the emerging industry.
But Mr McCallum, and other undersea experts like him, question the viability of the industry, fearing the only winners will be early investors and the companies selling the machinery needed to mine the ocean floor.
Technical hurdles
Commercial deep-sea mining is not yet legal but the International Seabed Authority (ISA) is in the process of finalising a framework that would govern the environmental, financial and regulatory requirements of the industry.
Under the draft regulations, companies would need to demonstrate they have the technical and financial capacity to carry out proposed mining operations.
"You're operating at pressures that are hundreds of times greater than atmospheric pressure on the surface, so as you go deeper, subsea engineering gets more complicated and incredibly more expensive," Mr McCallum said.
The nodules have to be physically gathered from the ocean floor by robotic collector vehicles before being transported to the surface through a riser system — like a massive vacuum hose.
No commercial-scale system exists yet, although industry leader The Metals Company successfully tested a prototype system in 2022, lifting more than 3,000 wet tonnes of nodules — including moisture soaked into the porous material — to the surface.
The company described the process as "a marriage of mining and offshore oil and gas technology".
But that amount was only a fraction of the 1.5 million wet tonnes the company forecast as its initial annual production rate in a 2025 pre-feasibility study.
Victor Vescovo, an American investor and technology developer with a background in metals processing, believes the targets deep-sea mining companies are forecasting are unrealistic.
"It's human psychology to see nodules lying on the sea floor and say wow, that's free money, we just need to pick it up," he said.
Like Mr McCallum, Mr Vescovo is one of the few people on Earth to have operated machinery at extreme depths, having personally piloted submersibles to the deepest points of all five oceans.
He said comparing deep-sea mining with offshore oil and gas was misleading because oil and gas operations did not generally occur below 4,000 metres.
"That pressure hits every square millimetre of anything you put down there and if these collector vehicles break, you've got to bring them all the way back up," he said.
"But because not many people have experience working at those depths, the investors and management teams of these deep-sea mining companies make optimistic assumptions about it."
In a comment to the ABC, The Metals Company said offshore oil and gas operations had reached depths of about 3,600 metres and deep-sea mining was therefore "not pushing the operating envelope significantly".
The company did not respond to the ABC's questions about the technical challenges or criticisms of the economic case underpinning the industry.
Declining demand for minerals
Two of the four key metals found in polymetallic nodules — nickel and cobalt — were once considered central to the electric vehicle revolution.
But more than half of the world's EV batteries are now based on lithium iron phosphate, or LFP, chemistry, which does not require either.
Chris Fleming, a professor of economics at Griffith University Business School, said it was no longer clear if deep-sea mining was needed for the electrification of cars.
"They are also working hard to get sodium-based batteries up and running, so if that's the way of the future, we could see the demand for these minerals fall off a cliff."
Other experts are optimistic.
Seaver Wang, an oceanographer and director of the US Breakthrough Institute's climate and energy program, said nodules had the potential to transform the global manganese market as battery makers turned to lithium-manganese iron phosphate chemistries.
He said about half of US cobalt demand goes to superalloys used in turbines, aerospace and defence.
"This is a rapidly evolving situation that the sea-floor industry has to answer for but if you ask me whether demand for those metals will continue to increase, the answer is yes."
He did, however, share concerns that industry forecasts might be overly optimistic.
In 2021, Belgian deep-sea mining firm Global Sea Mineral Resources lost its prototype collector after the cable connecting it to the surface disconnected, plunging the vehicle to a depth of 4,500 metres.
The firm managed to retrieve it, but Mr Wang said the incident illustrated the challenges the industry faced.
"If they lose 5 per cent here and there because of bad weather or equipment failure, that effect stacks and that's the hurdle that will make or break some of these companies," he said.
Despite that, Mr Wang was confident underwater technology was evolving at a rate that could make the industry viable.
"The opportunity to address three to four mineral constraints at the same time alone is worth policy attention," he said.
Mr Vescovo holds fears for the industry with mining companies taking billions from investors to fund commercial operations.
He thinks it will crash when the economics prove the industry is not profitable long term.
"But they'll be using investors' money, not money from their own operations and then one day the cheques will stop coming.
"That's how start ups fail, but it takes a while, especially if you have a really good story and you're able to raise a lot of money off it, which is exactly what they've done."
