Anta de Adreñones, located on top of a hill at 426 meters above sea level in the Sintra Cascais Natural Park, is believed to be a Stone Age burial chamber or rock monument, with a granite-stone structure and a portico about approx. It is 5 meters high with monoliths resting horizontally on vertical stones. It is believed that the passage was a mass grave or a dolmen during the Stone Age, although no artifacts or burial chambers were found to prove this.