A total of 999 stairs lead visitors through a massive hole in a mountain known as the Gateway to Heaven.

A tunnel dug through the side of a mountain by hand provides convenient yet death-defying access to a small village in China.

Spanning the El Chorro gorge, this pathway used to be one of the most dangerous crossings in the world.

A once grand hotel is being retaken by the wilderness it once capitalized on.

Abandoned city hidden from the world by tropical forest and isolation on the coast of Kenya.

A battle between nature and architecture in the Cambodian jungle.

Almost half-way between the Equator and North Pole.

A true treehouse near Hawai'i Volcanoes National Park.

The remnants of an early American attempt at missile defense.

Tour the world's largest deep-hole dimension granite quarry, where you can view the plant where gravestones are made and roll a ball down the outdoor granite bowling alley.

The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.

Virgil's many-mouthed gateway to the underworld.

Man-made cave with enigmatic carvings, some say made by the Knights Templar themselves.

Shakespeare, David Bowie, Dr. Who, the Saxons, Druids and Romans are all claimed to have all made their mark on these massive man made tunnels.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

Tubing through a cave lit with glowworms is a magical underground experience.

Stalactites and stalagmites in a beautifully lit cave.

A gargantuan crystal kingdom.

Slaughterhouse Five opens its dank doors to fans of Kurt Vonnegut.

America's greatest forgotten garden and the former stomping grounds of Son of Sam.

These azure waters were thought to lead to the Mayan Underworld.

The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

A peace symbol in Northern Ireland is among the world's largest hedge mazes.