The underground sanctuary of a mysterious ancient cult.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

This Venice bookstore has resigned itself to constant flooding by keeping its books in bathtubs and boats.

A 16th-century horror show built in a lovely Italian garden.

This abandoned mansion has a window for every day of the year, and is a temple for ghost hunters.

These "mermaids" that surround a plaza in Segovia are rather sphinx-like.

Containing a personal collection of more than 1,200 jugs from around the world.

1,300 chamber pots and their eccentric collector.

A site outside Pedraza, Spain where a plaque tells a tale of seven-legged creatures that were said to have once lived in the area—in the fictional world of Kcymaerxthaere.

The ruins of an ancient stadium that hosted gripping chariot and horse races.

This century-old ruined temple was reborn as "The Church of Skate."

Founded in 670, this ancient mosque is an architectural time portal.

The largest and best-preserved Roman amphitheater in Africa was designed to seat 35,000 people.

This newly renovated Soviet-era outdoor exhibition center is bigger than the principality of Monaco.

This 15th-century cave complex is one of the very few monasteries to stay open throughout World War II and the Soviet regime.

This remote lake in Siberia supposedly contains a fearsome sea monster.

Volunteer-run library dedicated to exploring five decades of activism history.

The world's oldest hologram gallery and the labyrinthine laser laboratory of a holography pioneer.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

The world’s oldest and biggest bookstore stocking only mystery, crime fiction, espionage, and thrillers.

This region of Slovakia boasts some of the most plentiful opal mines in the world.

A forgotten graveyard lost in the tangles of an overgrown garden.

Budapest's Communist-era statues, banished to the suburbs.

Museum dedicated to the terror regimes of Hungary.