The ocean floors are littered with polymetal nodules the main element in them is manganese more than humans could ever conceivable use billions of tonnes worth they form naturally overtime and are constantly being made and growing as see sediment covers older nodules new ones form on the surface of the sediments this also means there is layer upon layer of them under the seabed not just on the surface of the deep seabeds.
I thought that was a CIA hoax so they could use the Glomar Explorer to raise a sunken Soviet Submarine................
https://en.wikipedia.org/wiki/Project_Azorian
The recovery operation in international waters about six years later used mining for manganese nodules as its cover. The company was nominally owned by Howard Hughes, secretly backed by the CIA, who had paid for the construction of the Hughes Glomar Explorer.[5]