The largest self-built castle in the U.S., six decades in the making.

A lovers' beach, tucked below the surface of the island, provides a safe haven for romance.

This lower Manhattan bar is set up in an old bank vault that rests beneath a skyscraper that hides even more historic splendor.

The damming of the Colorado River buried an iconic geologic formation before record-breaking drought brought it back.

Most-visited and most-photographed slot canyon in the American Southwest.

Dramatic river bend surrounds a natural red-rocked pedestal.

This 38-drawer structure claims the superlative title, but it isn't the world's only giant filing cabinet.

Tour the world's largest deep-hole dimension granite quarry, where you can view the plant where gravestones are made and roll a ball down the outdoor granite bowling alley.

This cavernous abandoned marble mine is popular with ice skaters in the winter.

The glowing blue Nymphaeum of Emperor Tiberius.

Geothermal waterfalls in Tuscany.

Stalactites and stalagmites in a beautifully lit cave.

According to the local folklore, the myriad colors of this lake are the result of a lovelorn wizard's blunder.

Renaissance villa and gardens complete with a tormented colossus.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

An abandoned railroad tunnel is now filled with bioluminescent bugs.

More than five miles of underground passageways.

Enormous lake at the bottom of a unique cave system.

Visit this playground of waterfalls, caves and sinkholes.

Behind medieval walls in a corner of bustling Palermo is a breathtaking former mosque once inhabited by a wolf-taming hermit.

Every inch of this chapel is covered in craftsmanship from across the world.

This mosaic-lined medieval church was built by the world's first admiral and still holds mass in ancient Greek.

Cave shrine on Monte Pellegrino.

Thousands of well-dressed mummies below a monastery.