Tiny castle in the middle of the Rhine made you pay to pass.

The prototype for all Nazi concentration camps.

The footprint of a frustrated devil in Munich's Cathedral.

Daily jousts and dancing barrel makers in Munich's mechanical clock.

Go surfing on this urban wave, "hacked" by local surfers.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

Where commuters walk through remnants of an ancient Greek necropolis.

A 2,000-year-old computer demonstrates remarkable engineering and astronomical precision.

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

Giant acorn-shaped 19th century gazebo from which suburban Washingtonians gazed upon the original "silver" spring.

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.

A monument marks the location of the first successful steamboat demonstration.

A dramatic, 360-degree recreation of Pickett's Charge in the Civil War.

An 18th-century Spanish watchtower made entirely of seashell and concrete.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Concrete strongholds stand guard over the Golden Gate.

A giant Troll lurks under one of Seattle's oldest bridges.

Statue of Lenin, from Cold War era Czechoslovakia, now in a neighborhood of Seattle.

A museum dedicated to the history and exploration of both popular music and science fiction.

A wall of chewing gum moonlights as collective art.

This blowhole is supposedly caused by a trapped monster.

These ancient sacred grounds granted amnesty to those who managed to reach them.