Warning sign for deep and cold water at some fenced off gravel quarry and pits on 29th December 2021 in Berkswell, United Kingdom. A gravel pit is an open-pit mine for the extraction of gravel. Gravel pits often lie in river valleys where the water table is high, so they may naturally fill with water to form ponds or lakes. These areas are known to be dangerous places to play and hence cordoned off.