Once the path of a railway tunnel, now home to thousands of glowing insects.

The world's oldest known salt mine.

Home to Hollywood history and a very special salt crystal.

These numerous Israeli caves were dug out over hundreds years for reasons ranging from hideouts to burial sites.

The secret city built by the Manhattan Project.

A gargantuan crystal kingdom.

An underground city of salt.

Some North Carolina Cherokee have been keeping their history alive for decades with a single play.

An abandoned village that doubled as "The Hunger Games"' post-apocalyptic wasteland is up for grabs.

The highest point in Tennessee, Clingmans Dome bears witness to the ravages of one type of insect.

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

An abandoned stretch of highway, deteriorating next to the operational one.

An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

An unfinished rail tunnel that has since been used to house cheese and bats.

A retired tobacco farmer's pet project turned into a sturdy miniature city.

This short pedestrian span is also known as the "Fairy Walking Bridge" because it looks almost magical.

An enormous, inside-out glass globe built in 1935.

A historic observatory precariously perched at the "Top of Europe."

This amazing pedestrian suspension bridge in the Swiss Alps would give Indiana Jones nightmares.

The oldest bridge in South Carolina may have been designed by the same man who created the Washington Monument.

The tallest stationary dunes on Earth also happen to sing.

The second largest in the world, this massive underground void has its own subway.

An abandoned whaling station on an active volcano offers hot springs in Antarctica, surrounded by ruined giants of industry.

The former home of the Nobel Prize-winning American writer is now home to dozens of six-toed cats.