The transition from a global hydrocarbon economy to a green energy framework, coupled with the expanding middle class in developing nations, has precipitated an urgent demand for new sources of critical materials. While traditional terrestrial mining faces geological and geopolitical constraints, the deep ocean has emerged as a frontier for essential industrial metals. Polymetallic nodules and cobalt-rich ferromanganese crusts represent two of the most significant untapped resources, containing tonnages of critical metals that, in some cases, exceed global terrestrial reserves. The exploitation of these deposits involves complex technical challenges, significant environmental considerations, and shifting geopolitical strategies, particularly as nations seek to reduce dependence on foreign suppliers for minerals vital to national security and economic prosperity.
Geological Characteristics of Polymetallic Nodules
Polymetallic nodules, often referred to as manganese nodules, are two-dimensional mineral deposits that rest directly on abyssal plain sediments. These rock-like formations typically resemble potatoes in size and shape and are scattered across vast deep-water plains. They are found at water depths ranging from 3,500 to 6,000 meters. A prominent example of this environment is the region southeast of Hawaii in the Pacific Ocean.
The formation of these nodules is an extremely slow geological process. Metals enter the ocean through erosion of continental landmasses or from seafloor hydrothermal vents located in volcanically active areas. These metal ions attach to a nucleus, which may be a rock fragment, a shell, or other debris. Over millions of years, layers of metal oxides precipitate around this core, growing at a rate of only a few millimeters per million years. Consequently, larger nodules can be several million years old.
The primary composition of polymetallic nodules includes manganese and iron. However, they are economically significant for their enrichment of other critical metals, including nickel, copper, cobalt, and molybdenum. They also contain small amounts of rare earth elements and platinum. The Clarion-Clipperton Zone (CCZ) in the Pacific Ocean represents the largest known nodule field by both area and tonnage. Conservative estimates indicate that 21.1 billion dry tons of polymetallic nodules exist within the CCZ manganese nodule field. Based on these estimates, the tonnage of many critical metals contained within these nodules exceeds the reserves found in terrestrial mines globally.
Cobalt-Rich Ferromanganese Crusts
Distinct from nodules, cobalt-rich ferromanganese crusts are two-dimensional deposits that form pavements on rock outcrops. These crusts are located on the slopes and summits of underwater mountains, known as seamounts, as well as on ridges. They occur at water depths ranging from 400 to 7,000 meters.
The formation mechanism involves the precipitation of metals from surrounding seawater over millions of years. This process creates coatings of iron and manganese oxides on the substrate rock. The thickness of these crusts varies depending on the age of the seamount, ranging from a few millimeters to a few decimeters.
The economic value of these crusts lies in their rich composition of critical metals. In addition to manganese and iron, they contain significant quantities of cobalt, nickel, molybdenum, tellurium, platinum, vanadium, and rare earth elements. The Pacific Ocean Prime Crust Zone is identified as the area with the highest tonnage of these critical-metal-rich deposits. Estimates suggest that approximately 7.5 billion dry tons of cobalt-rich ferromanganese crusts occur in this zone. Similar to nodules, many of the elements contained within these crusts are estimated to be greater in total tonnage than those found in global terrestrial reserves.
Extraction Technologies and Technical Challenges
The extraction of deep-ocean minerals presents distinct technical hurdles depending on the type of deposit. Nodule mining and crust mining require fundamentally different approaches due to the physical nature of the deposits.
Polymetallic nodules sit loosely on soft sediment. This allows for the use of suction-based extraction methods. Tests in the Clarion-Clipperton Zone have involved machinery that suctions nodules from the seafloor and transports them to ships positioned above. More than 17 exploration licenses currently exist, primarily within the CCZ.
In contrast, crust mining is technically more difficult. Because crusts are firmly attached to substrate rock on seamounts, extraction requires separating the valuable mineral layer from the underlying bedrock. Successful mining operations must recover the crusts without collecting excessive amounts of substrate, as this would dilute the ore quality and reduce economic viability.
Underwater brine pools represent another potential source of critical minerals, particularly lithium and rare earth elements. These hypersaline brine pools are found on the seafloor in various parts of the world, with a notable concentration in the Gulf of Mexico. Lithium becomes concentrated in these brines over millions of years. However, harvesting minerals from these pools presents unique challenges due to their form and location.
Environmental Implications and Marine Life
The extraction of deep-sea minerals raises significant concerns regarding the impact on marine ecosystems. Deep-sea environments, particularly those in sensitive locations, are home to fragile and often poorly understood marine life. The fragility of these ecosystems means that mining activities could cause substantial damage to some of the least explored and least understood parts of the planet.
Seamounts, which are the primary locations for cobalt-rich ferromanganese crusts, host specific marine communities. Little is currently known about the marine life found on these seamounts, particularly in the regions most likely to be targeted for crust exploration and mining. The potential for habitat destruction and the long-term ecological consequences of removing substrate-attached crusts remain critical areas of scientific study and regulatory concern.
Similarly, the collection of polymetallic nodules from abyssal plains involves disturbing the sediment and the organisms that inhabit these deep-water plains. The slow growth rate of nodules and the specialized nature of deep-sea fauna necessitate careful environmental assessment before large-scale exploitation proceeds.
Global Reserves and Commercial Viability
The scale of deep-ocean mineral resources is substantial. The International Seabed Authority has estimated that polymetallic nodules could produce 20 million metric tons of cobalt alone. This potential yield underscores the economic significance of these deposits.
Despite the vast reserves, the commercial viability of deep-sea mining remains uncertain. It is unclear how much of these resources can ultimately be extracted in a way that is both commercially viable and environmentally sustainable at scale. Deep-ocean mining has not yet been carried out in the Exclusive Economic Zone (EEZ) of any nation, nor in areas beyond national jurisdiction. However, extensive mineral exploration and environmental studies are currently underway, and exploitation regulations are being codified. These activities indicate that mining operations are likely to begin in the near future.
Projections suggest that if deep-ocean mining follows the evolution trajectory of offshore petroleum production, approximately 35 to 45 percent of the global demand for critical metals could be met by deep-ocean mines by 2065.
Geopolitical Dynamics and Regulatory Frameworks
The geopolitical landscape of deep-sea mining is shaped by international agreements and national interests. The International Seabed Authority (ISA), formed in 1994 as an autonomous organization under the United Nations, holds the authority to regulate the exploration and extraction of mineral resources in seabeds beyond national jurisdiction. The ISA operates under the 1982 UN Convention on the Law of the Sea (UNCLOS). All parties to UNCLOS are members of the ISA and are subject to its rules and regulations, which mandate that exploration and extraction occur only under an ISA contract.
The United States is not a signatory to UNCLOS and therefore is not a member of the ISA. This distinction has influenced recent policy shifts. In April 2025, a presidential executive order signaled a clear change in direction, indicating that the United States is prepared to pursue deep-sea mining outside the regulatory framework of the ISA. The policy emphasizes the importance of seabed mineral resources in strengthening the economy, securing the energy future, and reducing dependence on foreign suppliers for critical minerals.
The United States controls seabed mineral resources in one of the largest ocean areas in the world. Existing authorities and international partnerships are being leveraged to access potentially vast resources in seabed polymetallic nodules, other subsea geologic structures, and coastal deposits. These deposits contain strategic minerals such as nickel, cobalt, copper, manganese, titanium, and rare earth elements, which are deemed vital for national security and economic prosperity.
In 2022, the ISA granted three five-year exploration licenses for polymetallic nodules within the EEZ. The first deep-sea tests for mineral extraction were performed by Japanese miners in their coastal waters, marking an early step in the practical application of these technologies.
Conclusion
The deep ocean holds immense reservoirs of critical metals that are essential for the transition to a green energy economy. Polymetallic nodules and cobalt-rich ferromanganese crusts offer reserves that, in many cases, surpass terrestrial equivalents. However, the path to exploitation is fraught with technical challenges, from the mechanical difficulty of separating crusts from rock to the environmental risks posed to fragile deep-sea ecosystems. Geopolitically, the sector is witnessing a divergence in regulatory approaches, with some nations adhering to the ISA framework while others, like the United States, seek to bypass it to secure strategic supply chains. As exploration matures and environmental understanding deepens, the balance between resource extraction and ecological preservation will define the future of deep-sea mining.