Manganese nodules are small, rounded concretions of manganese and other metals, such as iron, copper, and nickel, found on the ocean floor. They form by the slow precipitation of metals from seawater and are significant as potential sources of rare metals and for studying oceanic geochemical processes.
Reference: Glasby, G. P. (2006). “Manganese Nodules: A Review and Global Environmental Implications.” Oceanography and Marine Biology: An Annual Review, 44(1), 1-66.