More than 6 million users of X (formerly Twitter) have seen since yesterday a viral video of a 29-story windowless building in New York.

It has been described as one of the city’s strangest and most iconic skyscrapers, but little information has been released about its purpose.

Unlike the many neighboring residential and office buildings, it is impossible to peek inside. True to the original designs of the designers, there are no windows and the building is not lit.

At night it becomes a giant shadow that blends into the darkness while its large square holes emit a distinct, dull hum that is often drowned out by the sound of passing traffic and city sirens.

The NSA’s secret surveillance center

The mysterious and somewhat… dark skyscraper named Long Lines Building is not located in any of the well-known tourist spots of the city. It rises at number 33 Thomas Street, in Lower Manhattan, and belongs to the telecommunications company of New York, AT&T.

However, the 165-meter-tall granite skyscraper is also secretly the “headquarters” of the National Security Agency — a covert surveillance hub used to tap phone calls, faxes and Internet data — according to the Intercept.

Based on architectural plans of the building, public records and interviews with former AT&T employees, it appears that the 33 Thomas Street skyscraper has served as an NSA surveillance site, codenamed TITANPOINTE.

The skyscraper began to be built in 1969 and was completed in 1974. It has 29 floors without windows and three more underground levels.

The building was designed by the architectural firm John Carl Warnecke & Associates, whose grand vision was to create a communications nerve center like a “20th-century fortress, where protons and neutrons would replace spears and arrows and quietly lay siege to an army machines”.