One-time winter retreats of prolific inventor Thomas Edison and his friend Henry Ford.

Home to Hollywood history and a very special salt crystal.

The largest self-built castle in the U.S., six decades in the making.

Home of Howard Hughes' Spruce Goose.

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

The ghostly remains of an accidental sea.

Site of the first official transatlantic wireless communications.

The last holdout of a pre-Civil War rebel who took the matter of slavery into his own hands.

This pre-Revolutionary War grist mill ground corn and flour for soldiers in seven American wars.

America's post-nuclear-attack chambers of Congress.

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

An enormous warehouse filled with parade floats collected from over 50 years of patriotic celebrations.

Take a steam-powered ride straight out of the past on this Appalachian rail line.

The world's largest fully steerable radio telescope.

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

The site of the climactic scene from the classic horror film is now a historic landmark.

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

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

This pun was simply waiting to come to life, and one defense contractor made it so.

A secretive collection featuring objects that were designed specifically for U.S. intelligence operations.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

Collecting 700 years of mechanical death-dealing, the NRA's sprawling gun museum is a revealing testament to the American obsession with armament.