Quicksand

Quicksand is a colloid consisting of fine granular material and water. It forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form in standing water or in upward flowing water. In the case of upward-flowing water, forces oppose the force of gravity and suspend the soil particle. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluid-like texture. From Wikipedia