This grand gate once marked the entrance to a military barracks.

The ruins of an early 1900s neo-Baroque mansion sit nestled in a local park in the Hungarian plains.

This house covered in small stones was the work of one man over the course of nearly 30 years.

This psychedelic tourist trap is a leisurely descent into madness.

The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

Hamburgers are intensely proud of their tunnel beneath the river.

In Hong Kong, a fairytale flower tunnel is hidden within the gardens of a public housing complex.

The romantic botanic tunnel feels like a well-kept secret.

A repurposed railway tunnel with otherworldly acoustic characteristics.

The U.K.'s first underground railway line is now a lively thoroughfare for runners and cyclists.

This former train tunnel is filled with lines from a poem crafted by Robert Louis Stevenson.

This brick-lined pedestrian walkway was once a railway tunnel used during the Civil War.

Home to the longest wooden escalators in the world.

Once a sewer culvert, this road tunnel now offers cyclists and pedestrians a subterranean stroll under the C&O Canal.

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

This nondescript tunnel has been featured in a number of iconic movies, from “Roger Rabbit” to “Back to the Future.”

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

100 years later, they're still down there.

The first presidential bomb shelter was located in an old vault under the Treasury, connected to the White House via tunnel.

Seventy-five miles of limestone caves host more than a million bottles of wine beneath a sleepy Moldovan town.

Through the colossal entrance to this cave you'll find limestone tubs and an underground waterfall.

Approximately 300 million years ago, a massive coral reef existed in what is now the largest limestone cave in Japan.

The walls of these four caverns are filled with art that dates back to the earliest human presence in Europe.