Water
From AmtWiki
This rule is outdated
As it pertains to Monsters, often used in Quests
From the Rulebook
Denoted with silver tarp on ground or silver tape placed around the area. Any non-Aquatic, non-large, or non-flying creature entering this area must drop to their knees while moving through it.
From the Dor Un Avathar
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 which might force players to move slowly.