While we think of minerals and metal ores as being found in mines, that is, underground, another source of metal is actually underwater and not buried at all. Polymetallic nodules are odd objects that are found scattered on the ocean floor like pebbles. They're also known as manganese or ferromanganese nodules because of their high content in manganese and iron.

This is the typical metal content of a polymetallic nodule, according to the French Research Institute for Exploitation of the Sea:

Depending on the circumstances under which a nodule was created, the percentage of each metal can be lower or higher. It's thought that the metals accrete on the ocean floor as sediment and form around a nucleus made of a bit of solid debris such as a shark tooth or a fragment of a shell. They tend to be the size, shape and colour of small potatoes and simply lie as if strewn on the sea bad, no digging required. The time for a nodule of average size to form is in the range of 2-3 million years, though they've been observed to form more rapidly in the presence of concentrations of metal such as one would find near shipwrecks.

Their cross-section appears layered like in hailstones or geodes. They have a porosity of about 50% and can be found both in their standard form as well as small chains that have fused together like peanuts, or small clusters with multiple cores. Many theories exist as to how exactly they form but they're believed to be polygenic, another way of saying in various ways, but we're not sure how. Manganese is accreted on the ocean floor mainly from hydrothermal and terrestrial sources and a lot may be held by bacteria capable of accumulating manganese or iron. It may also be that they formed as carbon and calcium compounds dissolved, leaving the remaining crystalline or semi-crystalline metal ores in place. It's likely that all theories are correct.

Concentrations of these nodules exist in many sections of the ocean floor but the richest deposits so far identified are in the central Pacific and central Indian oceans. Deposits closest to the coast are found off the west coast of North America, off Japan, in the Caribbean and in the Arctic Ocean north of Svalbard, generally at depths between 3000 and 6000 metres. Now, the bottom of the ocean floor hundreds of miles from terra firma is a very inconvenient place for anything to be, so that's why they're still there and nobody has managed to devise an economically viable method of retrieving them. France, Russia, China and India have pioneered long-term research programs to explore the feasibility and reduce the cost of their recovery.

They were first found in the Kara Sea, off the coast of northern Russia, in 1868 and more were recovered from other oceans by the British research vessel HMS Challenger in 1876. For a long time after that they were merely scientific curiosa and not until 1957 did someone propose their commercial harvest. Because their metal content is not much higher than that of ore typically found on land, it's of more importance to countries which are poor in the minerals they contain. India, for example, has no domestic sources of cobalt, a strategic element used in durable alloys, and would benefit from technologies that would reduce its dependence on external sources.

So far no one has come up with a viable method of gathering polymetallic nodules. The quantity of metals in them, especially manganese, by far exceeds the known reserves on land but, until someone comes up with a practical way of getting to them or prices go through the roof and make it worthwhile, they'll stay right where they are.