Polymetallic nodules are a potential mineral resource for copper, nickel, cobalt, iron, manganese, and rare earth elements—metals that are becoming important for modern life, since they are used in making electronics like rechargeable batteries and touch screens, among other things.  

• These nodules are found in various deep ocean regions, including the deep Pacific and Indian Oceans. 

• The nodules of greatest mining interest are approximately potato-sized, and sit on the sediment surface across abyssal plains in the Clarion-Clipperton Zone (CCZ), a region spanning 5,000 kilometers (3,100 miles) across the central Pacific Ocean, at depths of ~4,000 – 5,500 meters (12,000 – 18,000 feet). 

Source: https://www.thehindu.com/sci-tech/science/why-is-india-pulled-to-deep-sea-mining/article28809029.ece