Researchers in the Netherlands and France studied quicksand, which is a combination of fine sand, clay, and salt water. At rest, quicksand thickens with time, but it remains very sensitive to small variations in stress – such as a human falling in it. At higher stresses, quicksand liquefies very quickly, and the higher the stress the more fluid it becomes. This causes a trapped body to sink when it starts to move. Instead, you need to lower the stress, which you can do by slowly trying to lie on your back.