Abstract: Mineral aggregates or nodules dispersed on the floor of an ocean are harvested and conveyed to a surface vessel, which tows a submerged harvesting apparatus. This apparatus is suspended from the vessel by a string of dual concentric pipes, which contains pressurized water for hydraulically conveying the aggregates to the surface. Pumps on the surface vessel force water down an annular passage between inner and outer pipes of the string. A portion of the downward flow is distributed to a feeder mechanism which collects crushed aggregates and feeds them into the return fluid flowing up the inner pipe. An inclined channel collects the aggregates from the underwater floor with water jets which dislodge and push the aggregates up the channel, where they are crushed by rollers. A portion of the downward flow of water from the surface is converted to lower pressure and higher volume downhole by using a dual piston converter to supply water to the water jets.