6. Holland Tunnel Ventilation Building
It is hard to miss the massive building that sits at the end of a long thin walkway that juts into the Hudson River just north of Tribeca. The windowless structure resembles a medieval defensive structure, but its true purpose has puzzled New Yorkers for years.
The building is actually an integral part of what occurs almost 100 feet below it: the Holland Tunnel‘s ventilation system, which transports the toxic carbon monoxide exhaust gas out of the tunnel. The tower is one of four vents, two on each side of the river, that house 84 fans—42 blowing and 42 sucking. Each tower is capable of fully refreshing all the air in the tunnel every 90 seconds when turning at full speed.