The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

An old hill fort covered in over 100,000 crosses is a symbol of Lithuanian nationalism and beliefs.

A historic scientific breakthrough that crosses nine borders.

First point in the Struve Geodetic Arc, a 19th century undertaking to measure the Earth's size and shape.

The enormous isokinetic structure hangs within a science center in Estonia.

The secret cells in the basement of the KGB's Estonian "Grey House" have been preserved as a grim Cold War museum.

For the last century, this international border within a small town has appeared and disappeared various times.

The world's first invisible city square is an homage to a man who pushed the boundaries of art.

Too much gossip or just missing curfew could land you in here for a couple of days.

A gorgeous stone viaduct crosses over a bucolic valley.

After 11 years of reconstruction work, Slovakia's finest hilltop chapel complex is finally reopen to the public.

An enormous and lavish Gothic church in Czechia.

A collection of modern and vintage pinball machines housed in the cellar of a 15th-century building.

Michael Jackson got Bad and Crocodile Dundee walked on heads in this iconic Brooklyn subway station.

This block in downtown Brooklyn was a hotbed of antislavery activism along the Underground Railroad.

The home of the first American-born saint is one of the only curved buildings in New York.

These desert alfalfa farms redefine the meaning of crop circles.

For centuries, Jeddah’s old town was a major trading port and a point of entry for Hajj pilgrims traveling by sea.

A 900-year-old village located in the highland of the Asir region that features historic architecture.

A historic structure that once housed pigeons used by farmers to help fertilize farmland.

A 19th-century fortress and collection of artifacts offer a tour of Saudi history.

These towering cliffs jut dramatically from the middle of a barren desert.

Within the Saudi Arabian desert lies a 2,000-year-old ghost town made of stone and mud.

More than 100 monumental tombs carved into rock are preserved at this ancient Nabataean site in the Arabian Desert.