"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

100 years later, they're still down there.

The oldest continuously operating fish market in the United States.

People escaping slavery may have hidden inside a corpse vault.

The first park built over a freeway is a brutalist masterwork.

The complex has been abandoned since the Croatian War of Independence, barring a stint as a set for 'Game of Thrones.'

The most unloved grain elevator in Brooklyn was a “Magnificent Mistake.”

This empty Victorian manse on Staten Island is said to be haunted but is actually a real life crime scene.

This abandoned subway station can still be seen on regular commuter trips.

During the holidays, a vintage subway train whisks New Yorkers back in time.

Home to the infamous "Typhoid Mary" and the worst loss of life in New York's history prior to September 11, 2001.

The haunting, dilapidated remains of a former poorhouse on Staten Island's Greenbelt.

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

Ride the subways of yesteryear.

A tunnel once abandoned now thrives with graffiti art as an off-limits makeshift gallery.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

A crumbling hospital from the 1850s on Roosevelt Island.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

Once the supplier for noteworthy projects like the National Cathedral, this old brickworks now lies abandoned.

A derelict bit of transportation infrastructure hidden in the woods.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

A subterranean shelter remains intact under a busy Paris railway station.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.