The Sun King's military models, saved from the Louvre.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

Paris' linguistic guide to the international language of, yes, love.

This massive library holds what was once the largest book collection in the world.

This tower is one of the best-preserved (and least-visited) remnants of medieval Paris.

The oldest stone house in Paris was built by its most famous alchemist.

A lone cannonball remains lodged in the walls of the Hotel de Sens as a sober reminder of a revolution.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

The remains of the medieval wall that once surrounded Paris with ties to the construction of the Louvre.

This modest marker near Notre Dame identifies the very center of Paris.

A former pillar of the Black community in Ellicott City.

Washed out by the Johnstown Flood in 1889, the surviving portion of the original two-span bridge now resides at Fishing Creek.

This three-story Victorian mansion is now home to the National Park Service’s Historic Preservation Training Center headquarters.

This anonymous businessman sculpted in bronze became an enduring memorial after 9/11, and had been mistaken by rescue workers for a survivor in the rubble.

This overlooked courtyard and museum celebrate Benjamin Franklin's life and achievements.

The only surviving 19th-century water-powered mill found in the Washington, D.C. area.

Built to defend Washington D.C. during the Civil War, this fort became a post-war nucleus for a thriving Black community.

At Washington's Dulles Airport is a satellite museum (no pun intended) with three quarters of a million square feet of aircraft history.

These unusual rooms on wheels are holdovers from the 1960s.

The narrowest house in America is seven feet of pure spite.

This brick-lined pedestrian walkway was once a railway tunnel used during the Civil War.

This rudder is one of the few remnants of a shipyard that once operated on the banks of the Potomac River.

America's first federal monument.

This tiny, one-acre cemetery serves as a reminder of when the Civil War got close to the nation’s capitol.