The only visible reminders of an elaborate engineering project that kept the tower from reaching the tipping point.

This network of roads is one of the most unique Etruscan creations ever discovered.

A dismembered holy head stares out from her beautiful reliquary at the St. Dominic Basilica.

The fishing boats that moor in this small lagoon enter and leave through a tunnel to the sea.

These temples are among the oldest free-standing stone buildings in the world.

This subterranean necropolis holds three floors of antler-dug grave caves.

This almost-forgotten underground dungeon housed several convicts over the years, including Caravaggio.

A centuries-old forgotten underworld lies beneath Malta.

After 400 years of being closed to the public, this monastery's underground tunnels and secret garden are now a museum.

The floor of this spectacular Baroque sanctuary is covered in memento mori.

Some of the tallest ancient American ruins continue to inspire awe and curiosity.

Looming communist pillar with a Czech twist.

The hidden lair of an alchemist rediscovered in a 2002 flood.

Two stunning 17th-century libraries and a cabinet of curiosities.

Macabre astrological automaton clock dating to the late 1400s.

This doorless 20th-century elevator doesn't stop or slow down.

A jaw-dropping brutalist space church seems to have crash-landed in the middle of America.

The pile of books behind the dignified writer gave the statue a most undignified nickname.

These medieval ruins have survived since the 12th century, even providing shelter from the Nazis.

The very center of Paris is surrounded by a bevy of strange good luck rituals.

Plot your trip through the city on the sidewalk.

Hidden in plain sight on a pair of Manhattan subway tunnels are a pair of public instruments that sing to each other.

A Pythagorean funhouse that tries, successfully, to prove that math is the coolest thing ever.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.