Quicksand
Someone stepping in it starts to sink. Around the person dense regions of sand and sediment form and grip the person. The viscosity of the quicksand seems to increase suddenly. It is now a gel. In order to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquify it.  The forces required to do this are quite large: to remove a foot from quicksand at a speed of one centimeter per second would require the same amount of force as "that needed to lift a medium-sized car".