The walls of America's oldest masonry fort famously "swallowed" cannonballs.

One of two identical monuments marking the sites of military training encampments in Peoria, Illinois.

A massive bur oak tree believed to be some 500 years old is the centerpiece of this quiet urban greenspace.

The world's third-largest scale model of the planets.

These Depression-era WPA sculptures have made a couple of stops over the years.

A collection of 11 million buttons illustrates the human toll of the Holocaust.

The final resting place of the world's first nuclear reactor.

The final resting place of up to 6,000 Confederate soldiers, the largest mass grave in the Western Hemisphere.

This modest post office sits on the site of H.H. Holmes' famous murder castle.

A tunnel with windows, so you don't miss out on the impressive views.

Free public park and gardens filled with peacocks and peahens.

One of the last surviving C-97s sits in a field in front of a Wisconsin inn.

A bypass decorated with large wooden troll carvings.

Software company Epic Systems has created one of the strangest and largest workplaces of all time.

A largely forgotten and touching neoclassical marble fountain overlooking a bear mound effigy.

66,000 pieces of art, donated by one of the world's richest men.

A once-booming Mississippi River town with a history of racial violence is now eerie and mostly abandoned.

The Denver airport is guarded by a 32-foot-tall sculpture of a demonic horse.

A 200-year-old bar in the historic French Quarter refuses to give up its place in history, nor its role in securing ours.

This remote island in Iowa was abandoned after being devastated by flood.

One of just a few cities in the world to base their center of government on an island. Others: Paris and Osaka.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

A measuring mishap led to these artificial stone ladies losing their stomachs.

This small museum in a 17th-century building honors three iconic Scottish writers.