After an first disturbance -- such as a person attempting to walk on it -- the water and sand in the quicksand separate and dense regions of sand sediment form ; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly . After an initial disturbance -- such as a person attempting to walk on it -- the water and sand in the quicksand separate and dense regions of sand sediment form ; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly .