The potential value of manganese nodules, also known as polymetallic nodules, has been known for a long time.
They were first discovered in the Kara Sea in the Arctic Ocean in 1868. A few years later, in the 1870s, a British Royal Navy ship, HMS Challenger, conducted a global marine research expedition and found such nodules exist in most of the world's oceans.
The Challenger was stripped of most of its cannon to make room for laboratories, a dredging platform and large quantities of rope.
It came across abundant deposits of the precious pellets while trawling the Pacific between Tahiti and Hawaii.
The prices of the metals in the nodules have only recently reached levels at which the considerable investment required to raise them would make commercial sense. Copper closed at $3.29 per pound on the US Comex on Friday May 24. On the same day, nickel was $6.68 a pound, cobalt was $12.75 a pound and manganese $2.10.
Technological advances have also made such mining feasible.
International attention is focused on the north-eastern equatorial Pacific, where there are dense concentrations of nodules containing particularly high levels of copper and nickel.
In some places they cover more than 70 per cent of the ocean bottom and are so densely packed the nodules touch one another.
"Three areas have been selected by industrial explorers: the centre of the north central Pacific Ocean; the Peru Basin in the south-east Pacific Ocean; and the centre of the north Indian Ocean," says the UN's international seabed authority (ISA).
Nodules are rock concretions formed of concentric layers of iron and manganese hydroxides around a core, like a pearl in a shell.
The core could be something such as a tiny fragment of shell or a shark's tooth or basalt debris.
They take millions of years to form, through a variety of chemical reactions involving metals in seawater, for example, through volcanic activity or dead plankton.
They can be found at any depth, even in lakes but the highest concentrations have been found in the deep ocean between 4,000 and 6,000 metres.
They vary in size from tiny particles to large pellets more than 20cm across. But most nodules are between 5 and 10cm in diametre.
Their surface is generally smooth, sometimes knobbly or otherwise irregular. The bottom, buried in sediment, is generally rougher than the top.
There are hills and cliffs in the seabed but researchers have identified wide areas that are flat enough for mining vehicles.
"A mine site was defined as a portion of the seabed where a commercial operation could be maintained for 20 to 25 years with a production of 1.5 to 4 million tonnes per year of 'good nodules,'" the ISA says.
Good nodules are defined as averaging at least 1.25 to 1.5 per cent nickel and 1 to 1.4 per cent copper, as well as 27 to 30 per cent manganese and 0.2 to 0.25 percent cobalt, it adds.