Nicknamed the 'Chestergate Hotel,' the underground tunnel had beds, lights, and even flushing toilets.

Once a passageway that led to the Shaniwar Wada palace fort.

The spectacular geological formations make it one of the most impressive caves in Europe.

A network of ancient tunnels and vaulted chambers with mystical Buddhist reliefs, built for ascetic training.

An immense cave that has served as an outlaw's treasure trove, an underground dance hall, a nuclear fallout shelter, and much more.

Unchanged for decades, World War II-era bullet holes and nostalgic ghost signs still haunt this desolate train station.

Besides being home to cave paintings and pre-Columbian Cuban archaeological remains, you get to take a boat through this stalactite and stalagmite covered cave.

Stuck in a bygone era, the oldest subterranean shopping street in Japan hides beneath the popular Asakusa district of Tokyo.

An underground world of gorgeous, palatial metro stations designed as a nuclear shelter complex.

The largest underground pedestrian tunnel system in the U.S. mostly caters to the people working in the offices above, but provides a great, air-conditioned way to traverse the city.

Once one of the most polluted caves in America, now an environmental success story and a museum dedicated to caves.

Underground attraction offering tours of the foundational ruins of a church nave that collapsed in 1674.

The warmest cave in the Ozarks features two seemingly bottomless lakes.

Hidden underground, a remarkable 10th-century bath complex survives virtually unaltered.