The glowing blue Nymphaeum of Emperor Tiberius.

Pompeii's less famous neighbor is still revealing her secrets.

Stalactites and stalagmites in a beautifully lit cave.

A submerged archeological park still holds the wonders of a Roman Sodom.

A sea cave with a waterfall and Viking past

These ancient sea caves hold Britain's highest concentration of Pictish rock carvings.

Now famous as the "Harry Potter bridge," this impressive concrete viaduct has long been an icon in its own right.

This series of Scottish caves has no clear purpose other than baffling historians.

360° views of the city plus five floors of optical illusions and hands-on science.

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

The highest point in Maine is a grand gift to the state citizens that also happens to be one end of the Appalachian Trail.

Paranormal experiences in this hotel's room 217 led Stephen King to write "The Shining."

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Mosaics, storks, and brothels—this ancient city has it all.