An encyclopedic collection of mammal penises.

A marvel of Byzantine engineering under the streets of Istanbul.

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

These stark white cliffs were spat up by the sea and are slowly being retaken by it.

As can be expected from a centuries old church built in a canal city, the undercroft of this house of worship is beautifully flooded.

This Tuscan lake hides a 13th century town that is still visible when the water recedes.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

A 2,000-year-old computer demonstrates remarkable engineering and astronomical precision.

This sculpture is an instrument played by the swell of the seas.

A musical instrument played by the sea.

Cold storage for agricultural biodiversity.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

The underground sanctuary of a mysterious ancient cult.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

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