10 stories under Manhattan's Grand Central Terminal is a network of activity.
Known as M42 and kept top-secret until 1980's, the hidden room powers Grand Central's train network with massive AC/DC rotaries and is not shown on any maps or blueprints. However, as large and impressive as these rotary converters are, they had one absurd achilles heel. They could be destroyed by throwing a bucket of sand into the rotating blades.
During World War II, 80 percent of troops and war material passed through Grand Central on their way up the Eastern Seaboard. Shutting down Grand Central would have been a major victory for the Nazis. It is for this reason that the area was kept secret for song long. Hitler eventually gained wind of this weakness from a former Grand Central worker who had expatriated to Germany and sent two U-Boats with four Nazi spies to destroy the convertors. The spies landed on Amagansett, Long Island, where they were seen by the coast guard before disappearing into the night.
Because of the sighting, the FBI was on the lookout for the spies and searched through everyone's luggage who passed through Grand Central, day after day, until they found a set of luggage they could have only belonged to the German spies. When the spies eventually came to pick up the luggage before heading to destroy the rotaries, they were promptly arrested.
The 102 year old computer system, built by Westinghouse and considered to be the first electric computer. Apparently, every year Apple executives visit it to pay homage.
600 million year old New York schist and bedrock.
Track 61, the subway track that goes under Grand Central to the Waldorf Astoria hotel and famously transported President FDR to keep his disability hidden from the public. It also transported many celebrities and officials, and served as a quick getaway for many Presidents from the hotel.
FDR's train, constructed to stabilize the side-to-side train movements that would plague FDR during train travel due to his partial paralysis from polio. The train could fit FDR's armor plated car, which would drive off the train, onto a platform, and straight in the elevator.