An architect's childhood fantasy treehouse made real.

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.

England's most notorious suicide spot.

The oldest working mechanical clock in the world.

A vast Communist-era exhibition space in the Bulgarian capital.

A magnificent, glittering building tucked into a tiny Bulgarian mountain town.

The remarkably well-preserved final resting place of a powerful ancient Thracian king.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

UNESCO-protected ancient burial ground.

This historic black and white monastery is one of Bulgaria's architectural jewels.

An abandoned saucer-shaped monument in the Bulgarian mountains.

Beautiful Austrian natural history museum with carved ceilings and row after row of ancient taxidermy.

Baroque celebration of the end of the Great Plague of Vienna.

A tropical paradise just a short trip from the opera house is filled with beautiful winged creatures.

The beautiful baroque library of the Hapsburg empire.

This mosaic dedicated to John Lennon was tended for years by a Beatles super-fan.

A popular European tradition makes its way to the states on one of America's most famous bridges.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

At Chicago's revered Field Museum, visitors are invited to learn about and literally explore DNA.

Years of archaeology, just chillin' by the water cooler.

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.