Mausoleum and Holy Seat of Antiochus I of Commagene.

Roman ruins top white travertine terraces formed by ancient hot springs.

Naturally protected city carved from a rock face.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

Iceland's largest lagoon is home to stunning multi-colored icebergs.

The walls of the iconic dome are hollow and have a secret stairway.

There’s a hidden military meme engraved on the World War II Memorial.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

New York City's smallest piece of private property.

Elevated freight railway turned wildly successful urban park.

Decades' worth of lost and forgotten objects make up the collection of this tantalizingly odd little museum.

After starting as a whimsical tourist attraction that eventually found its way to ruin, these renovated canals offer a lush and exotic locale on the Pacific Coast.

San Francisco's urban bison herd has persisted in the face of extinction, development, and disease.

A lovingly curated curio shop in which the taxidermy wear newspaper party hats, and peanut shells are spread across every surface.

An alien church provides much needed silence in the midst of a Finland shopping center.

A subterranean house of worship carved right out of the bedrock.

Bronzes and busts that once stood in the city's main squares are now relegated to a small hidden courtyard.

This natural history collection contains the bones of a circus elephant that seem to be chasing a whale skeleton.

The tiny, studious rodents greet visitors to the National Archives of Finland.

One of the best-preserved Roman amphitheaters is still used today, but for less brutal events.