Manganese nodule
Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core.  
The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization.  
When visible to the naked eye, it can be a small test (shell) of a microfossil (radiolarian or foraminifer), a phosphatized shark tooth, basalt debris or even fragments of earlier nodules.
Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimeters across.  However, most nodules are between 5 and 10 cm in diameter, about the size of potatoes.  Their surface is generally smooth, sometimes rough, mammilated (knobby) or otherwise irregular.   The bottom, buried in sediment, is generally rougher than the top.

# Growth and composition
Nodule growth is one of the slowest of all geological phenomena  on the order of a centimeter over several million years.  Several processes are involved in the formation of nodules, including the precipitation of metals from seawater (hydrogenous), the remobilization of manganese in the water column (diagenetic), the derivation of metals from hot springs associated with volcanic activity (hydrothermal), the decomposition of basaltic debris by seawater (halmyrolitic) and the precipitation of metal hydroxides through the activity of microorganisms (biogenic).  Several of these processes may operate concurrently or they may follow one another during the formation of a nodule.
The chemical composition of nodules varies according to the kind of manganese minerals and the size and characteristics of the core.  Those of greatest economic interest contain manganese (27-30 %), nickel (1.25-1.5 %), copper (1-1.4 %) and cobalt (0.2-0.25 %).  Other constituents include iron (6 %), silicon (5%) and aluminium (3%), with lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium, along with hydrogen and oxygen.

# Occurrence
Nodules lie on the seabed sediment, often partly or completely buried.  They vary greatly in abundance, in some cases touching one another and covering more than 70 per cent of the bottom.  The total amount of polymetallic nodules on the sea floor was estimated at 500 billion tons by A.A. Archer in 1981.  They can occur at any depth, even in lakes, but the highest concentrations have been found on vast abyssal plains in the deep ocean between 4,000 and 6,000 meters.
Polymetallic nodules were discovered in 1868 in the Kara Sea, in the Arctic Ocean of Siberia.  During the scientific expeditions of the H.M.S. Challenger (1872-76), they were found to occur in most oceans of the world. Nodules of economic interest have been found in three areas:  the north central Pacific Ocean, the Peru Basin in the southeast Pacific, and the center of the north Indian Ocean.  The most promising of these deposits in terms of nodule abundance and metal concentration occur in the Clipperton Fracture Zone of the eastern equatorial Pacific between Hawaii and Central America.

# Mining
Interest in the potential exploitation of polymetallic nodules generated a great deal of activity among prospective mining consortia in the 1960s and 1970s.  Almost half a billion dollars was invested in identifying potential deposits and in research and development of technology for mining and processing nodules.  These initial undertakings were carried out primarily by four multinational consortia composed of companies from the United States, Canada, the United Kingdom, the Federal Republic of Germany, Belgium, the Netherlands, Italy, Japan and two groups of private companies and agencies from France and Japan. There were also three publicly sponsored entities from the Soviet Union, India and China.
In the mid-seventies, a $70-million international joint venture succeeded in collecting multi-ton quantities of manganese nodules from the abyssal plains (18,000 feet, 5.5 km + depth) of the eastern equatorial Pacific Ocean. Significant quantities of nickel (the primary target) as well as copper and cobalt were subsequently extracted from this "ore" using both pyro and hydro methods. In the course of this 8-year project, a number of ancillary developments evolved, including the use of near-bottom towed side-scan sonar array to assay the nodule population density on the abyssal silt whilst simultaneously performing a sub-bottom profile with a derived, vertically-oriented, low-frequency acoustic beam.
The technology and art developed during the course of this project were never commercialized because the last two decades of the 20th century saw an excess of nickel production. The estimated $3.5-billion (1978 US dollars) investment to implement commercialization was an additional factor. Sumitomo Metal Mining continues to maintain a small (place-keeping) organization in this field.

# Legal developments
The promise of nodule exploitation was one of the main factors that led developing nations to propose that the deep seabed beyond the limits of national jurisdiction should be treated as a common heritage of mankind, with proceeds to be shared between those who developed this resource and the rest of the international community.  This initiative eventually resulted in the adoption (1982) of the United Nations Convention on the Law of the Sea and the establishment (1994) of the International Seabed Authority, with responsibility for controlling all deep-sea mining in international areas.  The first legislative achievement of this intergovernmental organization was the adoption (2000) of regulations for prospecting and exploration for polymetallic nodules, with special provisions to protect the marine environment from any adverse effects.  The Authority followed this up (2001-2002) by signing 15-year contracts with seven private and public entities, giving them exclusive rights to explore for nodules in specified tracts of the seabed, each 75,000 square kilometers in size.  The United States, whose companies were among the key actors in the earlier period of exploration, remains outside this compact as a non-party to the Law of the Sea Convention.
A company by the name of Kennecott Copper had explored the potential profits in manganese nodule mining and found that it wasn't worth the cost. Other than the environmental issues and the fact that the profits had to be shared, there was not any cheap way to get the manganese nodules off the sea floor.
In the meantime, interest in the extraction of nodules had waned.  Three factors were largely responsible:  the difficulty and expense of developing and operating mining technology that could economically remove the nodules from depths of five or six kilometers and transport them to the ocean surface, the high taxes the international community would charge for the mining, and the continuing availability of the key minerals from land-based sources at market prices.  The commercial extraction of polymetallic nodules is not considered likely to occur during the next two decades.