Authorities in Adur and Worthing are warning beach-goers about quicksands, after a report of the potentially hazardous phenomenon came into the local authority's coastal office.
Thursday's night's alert concentrated on Shoreham Beach, at the Beach Road section of the coast below the high water mark.
The soft sand is believed to have been caused by fast flowing tides affecting the water's edge.
According to the district and borough councils, quicksand can quickly form and then disappear if the conditions are right.
Although the local authorities asked the public to contact them by calling 01903 238977, coastguards have since reminded beach visitors that the 999 service will always receive emergency calls.