The cave is 14 km northwest of Tarsus, Mersin, and is believed to be a holy place for muslims and the christians both. Around 250 A.D. 

a greek ruler of Tarsus city gave warning to seven young people who had acknowledged to loving a single God and that they would be punished if they do not worship idols. 

Those 7 young people escaped from the ruler’s cruelty and took protection in this cave according to some myth, they slept over 309 years. The people, who ultimately understood that they were in the cave, closed the gate of the cave and left them to death.