Quicksand is a mixture of solid particles (like sand) and water. When your foot presses down on it, your weight squeezes the water out from between the particles. That leaves you trapped by mud that feels nearly solid. Check out this video, where the host walks onto what looks like solid ground, and sinks. Within a few minutes, he’s surrounded by a puddle of displaced water. Also, he is a little more trapped than he expected to be
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Photo by DaveBleasdale 
