Quicksand was a thick natural substance, partly made from sand, into which individuals could sink.
Clara Oswald got stuck in quicksand while exploring a planet with the Eleventh Doctor. The Doctor used his braces to drag the submerged Clara out from beneath the sand. (PROSE: Into the Nowhere)
Underneath the surface of Skaro, in the Valley of Rocks, the human Zec risked to sink in a quicksand and was saved by his companion Don. (COMIC: The Dalek Trap)