At 207 feet, one of the world's longest continuous escalators.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

Once home to seven different roller coasters, Glen Echo has undergone many transformations since its founding in 1891.

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

The world's longest known cave system.

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