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By ALISTAIR MACDONALD And EDWARD WELSCH

Mining companies are increasingly looking to the sea for new caches of natural resources. Toronto's Nautilus Minerals plans to scour the seabed, nearly a mile below the surface in the South Pacific. WSJ's Alistair McDonald reports.

A new breed of small, specialized mining firms plans to dive deep undersea in a quest for rich sources of metals and minerals, as technology and demand make the seafloor increasingly attractive as the next big mining frontier.

Previous efforts to dig for deep-ocean deposits fizzled as the reserves were too expensive to mine or technologically out of reach. Now, new advances in robotics, computer mapping and underwater drilling—combined with historically high commodities prices—are reviving interest.

To be sure, costs and the innovative nature of the projects means that marine mining in the near term is far from given. That was underscored Friday, when Nautilus Minerals Inc., NUS.T -2.65% long expected to be the first to mine, announced delays to its project on the back of funding troubles.

Still, a handful of Canadian companies are close enough that they are signing up customers and finalizing drilling schedules and budgets.

Vancouver-based DeepGreen Resources Inc. announced last week that Swiss commodities trader Glencore International GLEN.LN -0.52% PLC has agreed to buy half of the nickel and copper it plans to process from tennis-ball-sized nodules sitting almost three miles below the water's surface between Hawaii and Mexico. The nodules contain about 30% manganese, a metal used to make steel, along with cobalt, nickel and copper. DeepGreen hopes to begin production by 2020.

Diamond Fields International Ltd., DFI.T +9.09% of Vancouver, British Columbia, plans to be using a ship or platform fitted with a nearly mile-long hose to vacuum up fine silt suspended near the bottom of the Red Sea by 2014. The company says the silt contains copper, zinc, silver and trace amounts of gold.

Earlier this year, Toronto-based Nautilus signed up its first customer for high-grade copper and gold that it hopes to be mining almost a mile below the South Pacific, in several sites off the coast of Papua New Guinea. It is having a system of submersible vehicles built that will plow the seabed and send mineral-rich slurry into pipes for processing on the surface. It had expected first production by the end of next year, but on Friday said that it is "in dispute" with Papua New Guinea over funding and warned of a possible delay in funding for the project's main vessel.

Since last summer, the International Seabed Authority, an independent body set up by the United Nations to regulate mining in international waters, has signed four new contracts with groups looking to explore the ocean floor, says Adam Cook, an ISA marine biologist. That is a jump from the eight contracts previously signed, six of which were inked over 12 years ago, Mr. Cook says. The new contracts include agreements with state and private organizations from Japan, Korea, Russia and China.

"In time, I believe that oceans could provide the world's demand for metals in its entirety," says Steve Rogers, Nautilus Minerals' chief executive.

The recent undersea push comes as mining companies scour the globe for fresh reserves, having depleted much of the world's easy-to-access veins. As ore grades deteriorate onshore, the relatively rich grades thought to be waiting below the water are justifying the expected large costs and risks.

Nautilus says one of its prospective undersea deposits in the Pacific Ocean promises to yield ore with an average 7.5% to 8% of copper, compared with 0.6% at an average onshore mine.

Enlarge Image

Nautilus Minerals

A robotic arm takes samples from the mineral rich "black smokers" that form through volcanic activity underwater.

Given that so little is known of the ocean floors, scientists are reluctant to estimate overall resources. But last year, a team of Japanese scientists said that they found an estimated 80 billion to 100 billion metric tons of rare-earth deposits in the Pacific Ocean, or nearly a thousand times more than current proven recoverable onshore rare-earth reserves, as estimated by the U.S. Geological Survey.

Companies have been mining shallower waters for decades. De Beers SA, for instance, currently mines an annual one million carats of diamonds off the coast of Namibia. And oil companies have made huge strides drilling for hydrocarbons in super-deep waters around the world.

But previous attempts at deep-sea mining haven't yet proven technologically possible or economically viable, despite more than a century of experience. Efforts date back to the 1870s, when a British research vessel trawled up manganese nodules from a depth of almost three miles as part of a wider scientific study of the world's oceans.

Commercial efforts to raise manganese from the ocean floor in the 1970s collapsed amid technical difficulties, among other problems. Despite recent gains in technology, some mining analysts say the challenges ahead for economic production are still significant.

Costs are still far from clear, and the limits of today's advances in technology are still largely untested. Last week's announcement from Nautilus came as little surprise to analysts, who say delays are common in mining and particularly so in projects pursuing new avenues.

Nautilus had to put its plan on hold in 2008, amid uncertainty around the financial crisis. Still weak financial markets were blamed on Friday for a delay in funding from the company's German partner. The company is also in dispute with government of Papua New Guinea, which has granted Nautilus its mining lease and is co-funding 30% of the project. Papua New Guninea is being buffeted by political uncertainty, amid interparty squabbles over the premiership.

Meanwhile, environmental groups have raised concerns about the possible effect of deep-sea mining on aquatic life. While nations have specific regulatory authority over the seabed under their territorial water, that oversight diminishes farther out to sea. Miners counter the likely environmental impact is less than onshore mining, in part because it doesn't require building roads and is far from human habitation.

Raymond Goldie, a mining analyst at Toronto-based broker Salman Partners Inc., believes a deep-sea mine like the one Nautilus envisions will still see delays. But he says it is a realistic project, which the mining world is taking seriously and watching with interest.

"These guys are a test case," he says.

Write to Alistair MacDonald at alistair.macdonald@wsj.com and Edward Welsch at edward.welsch@dowjones.com