Travel through the hidden underbelly of the city in colorful tunnels filled with art and history exhibits.

A Texas-sized particle collider that was abandoned due to its Texas-sized budget.

A railroad tunnel that, upon its completion, was considered one of the engineering marvels of the modern world.

Deep under Manhattan is the largest construction project in city history. So far, the project has taken the lives of 23 workers, or "a man a mile."

Visit the site of a subway powered by nothing more than air--and lots of it.

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

This network of tunnels housed an entire village for years, and even had a hall for screening movies.

The largest hydroelectric project in Laos created this eerie landscape.

A boat ride through utter darkness, lit only by headlamps, reveals an emerald green pool locals believe is sacred.

World's largest cave was only discovered in 1991, and now offers tours to the public.

This otherworldly church spire reaches up toward the heavens.

This army of concretions is an unexplained geological phenomenon unlike anything else in the world.

The longest continuous lava tube in Oregon.

Massive crater in central Oregon.

Sunset-colored dunes of volcanic ash make taking a poor photo impossible.

Trails through lava-carved tube-caves display sparkling bacteria and a rich history.

The deepest lake in the United States, and once the site of epic destruction that lives on in legends.

This strange hill on Old Fort Road where gravity works in reverse has been fascinating locals for years.

Nature’s skyscraper launches out of the terrain with attitude.

This volcanic cave lake is essentially hard proof that Iceland is the most magical place on Earth.

A popular Icelandic children's story comes to life inside this cave.

America's smallest "national forest" has only 33 trees.

Some call this eerie forest of crooked trees the "Bermuda Triangle of Romania."

This bone-filled cave got its name after they found 140 bear skeletons inside.