A massive volcanic plug is now a protected area that hosts a number of bird species.

Graffiti from the 16th and 17th centuries cover the walls of this part of the Tower of London.

Tucked away in the Tower of London is the weapon once used to execute high-profile prisoners.

The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

Richard III supposedly disposed of his nephews' bodies here in an effort to seal his claim to the throne.

A memorial for public executions that dates back to the 14th century.

Six ravens are kept captive (but well-fed) at the Tower of London to prevent the fall of the Crown.

A replica of the steam engine that hauled Queen Victoria's Royal Train is on display at Windsor.

Beneath the southern end of the Golden Gate Bridge is the “Gibraltar of the West Coast,” a fort built to protect the San Francisco Bay from naval attack.

Naturally carbonated water fills the mineral baths at this 19th-century California resort.

One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

A beautiful geyser under environmental threat.

The first park built over a freeway is a brutalist masterwork.

A sliver of nostalgia in Seattle's Viretta Park.

Exquisite irises bloom every spring in this hillside garden.

The arch that celebrated the completion of the Lincoln and Victory highways.

These markers are used to determine the ground stability of the Strip.

A Norwegian-born sculptor created these soaring guardian angels at the entrance to Hoover Dam.

This immigrant bridge once spanned the Thames but is now a fixture of an Arizona city.

Electrical pylon in the shape of Disney's beloved mouse.

This giant shopping mall still holds signs of its automotive past.

Thousands of crosses and markers for soldiers who lost their lives in Iraq and Afghanistan blanket a California hillside.

More than 700 species live within this murky marvel of a swamp on California's coast.