This is not your typical jungle gym.

Curious cave that stays a cool 56 degrees all year long.

Community bookshelf of 25-foot-tall book spines.

The history of the treatment of mental illness, illustrated in all its gory detail, and housed in an asylum.

The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.

Believed to be the remains of the lost Temple of Minerva, this nymphaeum has been known by its misnomer to this day.

2300-year-old Roman road connecting the empire.

Before it was buried by volcanic ash, this fast-food restaurant fed the hungry hordes of ancient Rome.

A special area of the National Archaeological Museum of Naples is located inside a metro station.

Belfast's namesake river still winds below the city center, hidden from sight.

These engineering masterpieces are a dominant feature of the Belfast skyline.

It was once nicknamed "the most bombed hotel in Europe."

The last Victorian market in Belfast.

A giant sculpture of the legendary "Salmon of Knowledge" celebrates the return of fish to Belfast's River Lagan.

The ship built to ferry passengers and goods to the doomed Titanic is now the last remaining White Star Line vessel.

For part of the year, this innovative greenhouse is painted white to keep the plants from getting scorched.

A museum marks and documents the site of the Titanic's launch.

This short bridge is the only thing connecting a tiny Irish island to the mainland.

Northern Ireland's Brobdingnagian stepping stones.

A creepy sculpture of a decapitated head invites viewers to see St. Louis through its empty eyes.

A historic house in St. Louis was plagued by a series of suicides committed by family members involved in a successful brewery.

Every day is like Christmas on this street where the traditional Neapolitan nativity scene comes to life.

This ancient market is often referred to as the oldest shopping mall in the world.

Legend has it this fountain built to symbolize the Pope’s power hides a strange secret concerning Neptune's manliness.