This unique structure allowed boats to bypass underwater mine barriers in the channel to enter the harbor.

This natural crater along the coast of Jersey may take its name from the devil statues who have watched over it.

Along the most northwestern point in Africa is an ancient cave surrounded by legends.

This monastery built into a cliff hides a perfectly preserved hermit in a glass coffin.

Nearly 40 miles and 1,000 years of limestone mines under Denmark.

Roman ruins top white travertine terraces formed by ancient hot springs.

Iran's colorful terraces are a rare geological masterpiece.

This landmark church concealed a crypt packed with bones that was discovered thanks to the London Blitz.

Nine caves filled with recently-discovered archaeological finds under the houses and the streets of the most ancient district of an old town.

A completely underground tour following the path of the most precious element: water.

The largest underground lake in Europe.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

Drink and dine in a coral cave that's at least 120,000 years old.

The short hike is worth the stunning views and crystal blue waters of a secluded beach.

Thousands of effigies of Buddha watch from caves set high above a legendary river, plus whiskey.

This winding road through an Italian canyon may be the most scenic drive in the world.

A maze of underground tunnels beneath one of the most beautiful forts in Serbia.

The largest spring in Northwest Arkansas is also home to a sprawling botanical gardens and a historically significant bluff.

This restaurant serves wood-fired fare served in a natural cave with a live spring.

The “Grand Canyon of Maine” is a three-mile gorge full of striking geological features.

The seventh largest spring in the world.

This cramped crevasse saw the entire population of its island suffocated inside its depths.

Legend says this cave housed a 16th-century cannibal and his family of 48.

Near this coastal cave stands a small memorial to the hermit who made it his home for three decades.