The Caves (or grottoes) of Hercules are a popular tourist attraction just outside of Tangier, Morocco in Cape Spartel. According to Greek mythology, Hercules is believed to have slept in these caves before attempting one of the 12 labors given to him by King Eurystheus of Tiryns. Story has it that it took Hercules 12 years to complete the labors.
Inhabited since prehistoric times, Fodor’s Travel adds:
The caves were used more recently to cut millstones, hence the hundreds of round indentations on their walls and ceiling otherwise attributed to Hercules’ clawing fingers. The caves are known for their windowlike opening in the shape of the African continent, through which the surf comes crashing into the lagoon and lower cave. Legend has it that the cave leads to a tunnel that crosses through the Strait of Gibraltar, leading you to the other side of the Mediterranean. [source]