Sentence:  [W]e have a surface current of saltish water from the poles towards the equator, and an <WSD>undercurrent</WSD> of water, saltier and heavier, from the equator to the poles. This <WSD>undercurrent</WSD> supplies in a great measure the salt which the upper current, freighted with fresh water from the clouds and rivers, carries back. 
Meaning of the word marked with <WSD> tags:  A current of water which flows under the surface, and often in a different direction from surface currents.