You can row a boat through this ancient underground reservoir.

Disused seaplane hangar gets a second life as the jaw-dropping home of Estonia's museum of maritime history.

The first tunnel to be dug beneath a river, repurposed as an art and museum space.

Elevated freight railway turned wildly successful urban park.

This isolated airport in the middle of the Everglades sees more alligators than planes.

This amazing ancient cave was kept secret for seven years to protect it from the public.

One of the last remaining covered bridges in the province of Ontario.

Hidden high on a remote mountain peak is a humbling and mysterious Vietnam War memorial.

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

A mapping error in the 19th century made loggers think this section of the forest was underwater.

A historic holy well hidden near a charming English park.

Known as the Royal Palace of the Euganean Hills, this often overlooked castle has been home for multiple noble families over the centuries.

An ancient stone fortress considered by many to be the eighth wonder of the world.

An Aztec temple, the largest man-made pyramid in the world, sits buried in earth with a Spanish church set on top.

This fairy-tale castle would make Dracula (who is said to have been imprisoned there) jealous.

This quaint Italian town keeps losing ground as the sides of its plateau slowly erode.

This dazzlingly turquoise mountain lake was created in the wake of a natural disaster.

The longest living non-clonal organisms on Earth.

This curious fresh and salt water lake holds a secret that draws in deep divers from around the world.

A hidden "place of interment for gentlemen."

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

Preserved Inca city and site of the last Inca stronghold against the Spanish conquistadors.

Defying all odds, this lonely little tree found a home on a log sticking out of a Canadian lake.

The world's longest and deepest train tunnel.