Which ocean resource is currently not economically profitable to mine? gravel sand manganese nodules salt?

Manganese nodules are an ocean resource that is not currently economically profitable to mine. These nodules, about the size of a potato, form on the bottom of the ocean and in addition to manganese contain copper, nickel and cobalt. The main reason they are not economically viable at this time is that there are already plenty of these metals to be found on land.