Two New York rooftops are home to the largest rooftop soil farms in the world.

The site of London's first coffee house has been serving refreshments of one kind or another for 360 years.

Eighty million natural history specimens call this gargantuan museum home.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

Several plaster noses are hidden in plain sight around London's Soho neighborhood.

Rebuilt remains of a temple to Roman god Mithras.

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

This collection of illusory paraphernalia has been culled from the history of London's premier magician's club.

A kaleidoscopic warehouse-maze of handmade neon signs.

Manhattan's lighthouse, erected at the insistence of the unsinkable Molly Brown.

There’s a secret bunker inside the Brooklyn Bridge, but the city won’t say where.

Leaf through over 30,000 artists' sketchbooks in what may be the world's largest collection of doodles

Headquarters of the world's fourth largest Hasidic sect and the first of many "770s" around the world.

Fabrication workshop where the encasement for One World Trade Center's beacon was constructed.

Gang fights don't just happen on the streets, they happen under them too.

Volunteer-run library dedicated to exploring five decades of activism history.

A museum devoted to bringing to light forgotten or neglected histories through exhibitions, education and public programming.

This innocent-looking townhome is actually a subway exit.

Neon signs that grace the windows of Bergdorf Goodman's and Tiffany & Co. come from this studio in Brooklyn.

Abandoned power station once home to a squatter community, now being developed into an arts center.

A huge resale shop for the cast-offs of the film & television industry.

In the middle of downtown Brooklyn lies a 165 year old secret hidden right below the surface (closed to the public indefinitely).

Underground vaults of a former brewery beneath Brooklyn's Crown Heights are now used to ripen cheese.

Risk arrest while partying to fund art.