What ocean resource is currently not profitable to mine?

Manganese nodules are small, round, black rocks that form on the ocean floor. They are composed of various metals, including manganese, iron, copper, and nickel. Although manganese nodules are a potential source of these metals, it is currently not profitable to mine them. This is because the nodules are located in deep water, and the cost of mining them would be high. Additionally, the environmental impact of mining manganese nodules is not fully understood, which could pose risks to marine life and ecosystems.