The world below the brine,
Forests at the bottom of the sea, the branches and leaves,
Sea-lettuce, vast lichens, strange flowers and seeds, the thick
 tangle openings, and pink turf,
This clipped line provides one of the few chances for the reader to draw breath, in a poem which ecstatically celebrates the wonders of nature simply by enumerating them. 
 “Brine” is a solution of salt and water, but here simply means “seawater”– Whitman is describing the aquatic world. Seawater is around about 3.5% salt, at the lower end of the concentration spectrum for brine, which ranges up to 26%, when the water becomes saturated. 

World Below the Brine