From AmtWiki

A Terrain Type as described in the Dor Un Avathar X and Nautical Nights

Denoted with silver tarp on ground or silver tape placed around the area. All creatures entering this area must drop to their knees while moving through it unless they are Aquatic, Large, or flying. These rules can also be used to simulate wading through muck and mud, dense foliage, or any other condition that might force players to move slowly.

See also