Millions of years ago, precious gem stones from the heart of southern Africa washed westward along the Orange River and emptied into the Atlantic Ocean.

While some swept up on the beaches along the Namibian coast, spurring a diamond rush in 1908, others came to rest on the ocean floor. For a little while at least.

Today, six ships comb the ocean, sucking sediment from the seabed. The immense vessels are operated by Debmarine Namibia, a joint venture between the Namibian Government and diamond giant De Beers.

Last year the marine diamond mining company produced 1.378 million carats of diamonds, at a time when the country’s land operations are waning.

To ramp up production, Debmarine Namibia plans to construct a $142 million ship-cum-tanker, which it says will be the world’s largest custom-built diamond mining vessel, measuring 577 feet long. It hopes this new vessel will join the fleet by 2021.

This type of vehicle – called a crawler ship – has a 280-ton mechanical arm that moves in a horizontal arc, dredging material from just beneath the sea floor, at depths of around 400 feet.

Diamonds are then sifted from the dredged gravel in a sophisticated treatment plant onboard the ship. The gravel is returned to the ocean and the gems are securely sealed in containers, loaded into steel briefcases, and flown by helicopter to shore.

No human hands touch the diamonds during the entire production process at sea.

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Gems in a haystack

Debmarine Namibia has a license to operate off the coast of Namibia until 2035 within a 2,316 square mile area – just under half the size of Jamaica.

But, while the ships mine 24 hours a day, 365 days a year, they’re not scouring every last square mile, explains Otto Shikongo, CEO of Debmarine Namibia.

“You only mine areas which are mineable and profitable,” Shikongo tells CNN. “It doesn’t mean that every place that you find diamonds you go mine.”

He says Debmarine Namibia has depleted a total of 50 square miles since production began in 2002, just two percent of the license area.

“The resource is patchy and not homogenous,” he says, adding that the future of the mining will depend on their understanding of the seabed and technological advances.

“It’s not the same as a land-based resource which you can see with your eyes … this one is 120 meters [393 feet] under the water,” he says.

Every so often Debmarine Namibia sends out unmanned, autonomous vehicles – much like underwater drones – to survey the seabed using sonar technology.

The team also makes use of a two-person submarine to examine the geology of the seafloor.

Next, a 12,000 ton exploration vessel scoops large samples from areas that are believed to hold diamonds.

To date these combined technologies have identified a mineralized area – or an area containing diamonds – of 617 square miles. This makes up just over a quarter of the total license area.

The hope is to discover more diamond-containing areas through further exploration and sampling, explains Shikongo.

Survival of the fittest

While marine diamonds may be difficult to find, they’re certainly worth the hunt.

Shikongo explains how nature ensured that only the “fittest” diamonds survived the journey along the Orange River, while weaker, imperfect stones were destroyed.

“Because the diamonds went through a high energy process, almost like a tumbling effect, only the best, high quality diamonds survived and made it to the sea,” he says.

As a result, Shikongo estimates that 95% of diamonds recovered from the sea are of “gem quality,” compared to just 40-60% of diamonds from land operations.

Read more: What’s behind our obsessions with gems?

Environmental cost?

But, in the search for these precious gems, thousands of tons of sediment is dredged up and then dumped overboard.

Marine scientists argue that seabed mining degrades the marine environment, potentially impacting marine species.

“The waters off the coast of Namibia are an important area for a high diversity of resident and migratory species, such as sharks, whales, dolphins and seals,” Kirsten Thompson, a marine scientist from the University of Exeter, tells CNN over email.

“Marine mining removes parts of the seabed with heavy machinery and habitat recovery from this type of disturbance can take decades.”

But Shikongo says Debmarine Namibia continually monitors its footprint.

According to Shikongo, the company’s environmental monitoring program found that it takes between two to 10 years for the seabed, and associated marine life, to recover.

However, in rockier terrains natural recovery could take more than 10 years.

Thompson adds that diamond mining can impact ocean life through increased ship traffic, noise, light and pollution, and is just one of many activities degrading the marine environment.

“Marine species are already experiencing profound changes due to climate change and other human-related activities, such as fishing, plastic pollution and shipping,” she says.

“Some species and habitats are simply not resilient enough to cope with these multiple stresses simultaneously.”

Offshore mining is increasingly important for the diamond industry in Namibia, as land-based production begins to tail off.

“Land operations have been there since 1908,” explains Shikongo. “So these resources are not finite as we know and as time goes … production tends to taper down.”

Debmarine Namibia commenced marine mining operations in 2002, with a haul of approximately 500,000 carats. At the time, sister company Namdeb’s land operations were producing around a million carats.

But over the years the tables have turned and marine operations now account for close to 75% of total diamond production in the country, according to Namibia’s Chamber of Mines.

With all eyes on the Atlantic, the question remains: How soon will we exhaust this precious underwater resource?

Shikongo believes there are probably enough stones scattered across the Atlantic seabed for Debmarine Namibia to continue mining for at least the next 20 years.

The only challenge is finding them.