Quicksand
Water circulation underground can focus in an area with just the right mixture of fine sands and other materials such as clay.  The water moves up and then down slowly in a convection-like manner throughout a column of sand, and the sand remains a generally solid mass.  This lubricates the sand particles and makes them unable to support any significant weight, since they move around with very little friction. They behave more like a liquid when exposed to stress.  Since the water does not usually go all the way up through the sand, the sand above does not appear to move at all, and can support leaves and other small debris. This makes it difficult to distinguish quicksand from the surrounding environment.