Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

A 140-year-old castle with underground caves hidden in San Francisco.

This hilltop park offers magnificent views and a Frank Lloyd Wright masterpiece.

A little barn in the middle of the city preserving Hollywood treasures since 1985.

Once strictly off-limits, this lower level housing curious and priceless vehicles of the Petersen Automotive Museum is now open to the public.

Shop selling 500 flavors of soda has something for everyone.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

The ruins of a Duke's bucolic dream, where camels once roamed alongside "Dutch" windmills and faux-Italian vineyards.

A majestic lion statue guards this Parisian botanical garden, and feeds on an unsettling item.

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

Commemorating the spot where the famous scientist fed his pigeons.

The New York hotel where tragic visionary Nikola Tesla spent his final hours, destitute and alone but for the pigeons.

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

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

An uber-secure, windowless tower of doom in the center of Manhattan is an NSA spyscraper.

This ruined fort hidden at the top of Central Park can't seem to keep from losing its historical plaques.

An unassuming secret door once led to a legendary watering hole.

The oldest family-run luncheonette in New York, last renovated in 1948, still serves food and drinks the old-fashioned way.

A three-story copper skybridge connects two Manhattan buildings with Art Deco luxury.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

The oldest surviving bridge in New York City, which reopened to pedestrians in 2015.

This artful dam holds back a crater-made reservoir that can be seen from space.

Unlike many reclaimed, abandoned mines, this Canadian cave system is still just a dangerous maze.