Abandoned railway line circling the city of Paris.

This sprawling Jewish cemetery is one of the largest in Europe.

These former Nazi anti-aircraft towers offer a singular view of the north of Berlin in the summer, and a sanctuary for bats in winter.

This massive stained-glass masterpiece is thought to be the largest Tiffany dome in existence.

Enormous lake at the bottom of a unique cave system.

Eighteen miniature death-scene dioramas.

James Turrell's total color immersion art installation, hidden on the top floor of a luxury handbag shop.

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

A hand-built folk art installation covered with messages of God's love.

An Australian lake whose pink hue defies scientific explanation.

This massive library holds what was once the largest book collection in the world.

Famously misunderstood wonder of the ancient world

The haunting "Hell House Altar" is one of the few remnants of an abandoned college that has been a magnet for local legends.

The only theatre in the world constructed specifically for sound movement.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

Resting place of Hollywood's immortals is also site of summer film screenings.

Hidden beneath the streets of London is a secret subway that only carried mail.

A walk through the historic city center, under the streets of Seattle.

This secluded aqua waterfall in the Grand Canyon is the perfect swimming hole, and the Havasupai tribe's fiercely protected natural wonder.

Under the right circumstances, this waterfall in Yosemite briefly turns a luminescent fiery orange.

Notorious artist squat renovated into legal studios.

A theme park that suffered massive flooding during Katrina was abandoned, and now proves irresistible to trespassing explorers.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.