 According to the Bulgarian archaeologist Alexander Fol some of the Thracian womb-caves had the property of letting the sunlight in only at certain times of the day, a natural phenomenon seen by the Thracians as acts of symbolic fertilization of the Earth womb or the Mother Goddess by the sun phallus of the Sun God.The site is an open-air museum maintained by the Burgas Historical Society.It is visited annually by 40,000 tourists.