Nestled in an unassuming residential area, this "forbidden forest" is rumored to spirit away those who dare to enter.

148 feet long and in the middle of a verdant forest.

Nature took her revenge on the "Wickedest City in the World."

The fishing boats that moor in this small lagoon enter and leave through a tunnel to the sea.

The oldest still-operating tavern in the United States was once owned by a pirate.

This large copper bolt represents the benchmark for measuring sea levels in most of Europe.

Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

This impressive collection of rare books on ancient mysticism inspired author Dan Brown.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

The attic of a four-century-old canal house hides a clandestine church.

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

Established in 1638 to battle the Black Death, Hortus Botanicus Amsterdam is a treasure trove of rare flora.

A floating cat sanctuary on an Amsterdam canal.

For 60 years, the academy had no permanent location until members voted Washington D.C. as its forever home.

There’s a special club house at the top for the bell ringers.

In the basement of the U.S. Capitol Building is a hidden storage room full of ornate floor tiles leftover from the 1850s.

Home to a vast and influential collection of Shakespeareana.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

A nostalgic bibliographic gem.

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

This lonely waterfront memorial to the men of the Titanic was erected by the "Women of America."

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