Home to the largest urban bat colony in North America, estimated at 1.5 million bats.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

Less a museum than a bizarre fantasy world created by artists and engineers.

Hidden beneath a bridge, this waterfall is known for the large amounts of graffiti painted on its base.

One of the oldest used bookstores in the U.S. has been selling antiquarian treasures since 1825.

Quite simply the largest tree in the world (by volume).

This whimsical group of statues is a favorite of children who love to climb all over Lewis Carroll's beloved characters.

The world's longest known cave system.

Remants of foundations, streets, and streetcar tracks inhabit the cliffs where a landslide occurred in 1929.

Crystal clear lake with a shoreline of travertine located in a Colorado canyon.

An artsy hidden staircase leads to breathtaking views of San Francisco.

Holy Grotto of Montserrat Monastery holds one of Europe's mysterious black Madonnas.

This colorful statue welcomes visitors to a Barcelona park.

Saint Joseph was said to heal the deaf and blind, but it was a much smaller miracle that got him canonized.

This former Baroque palace on the historic Calle Montcada is a rare architectural gem and was once the meeting place of an aristocratic scholarly society.

This cafe offers a curated collection of treats and tipples made by monks and nuns.

This wild explosion of color on a disused water tower tells the whimsical tale of a curious puffin.

The best preserved Paleochristian basilica in Rome hides a number of oddities and secrets.

A roadway designed for a visiting monarch.

The fierce beast perches atop a pedestal marking where the historic City of London gates once stood.

This otherwise ordinary close on the Royal Mile constituted the end of the world for Edinburgh’s poorest residents.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Historic fortifications form the boundary of Vatican City, but it’s not entirely surrounded by walls.

A series of three shiny letters mark the boundary of an old refuge for those being pursued by debt collectors.