Deep under Grand Central Terminal, beyond basements and subway lines and through 10 stories of bedrock lays a secret underground chamber that's almost as big as Grand Central Terminal itself. What's inside the room? A machine the Nazis wanted to destroy in World War II.
More specifically, the machines were rotary converters. There were 9 of them inside the underground room with each weighing 15 tons. They were able to transform 11,000 volts of alternating current to direct current to power all the trains above. Basically, as the Science Channel explains, they were the beating heart of the entire northeastern United States metro rail network (spanning over 2000 miles long).
When the US decided to join the Allies in World War II, Hitler recognized the importance of this underground room and sent German Nazi soldiers to destroy it. All they needed to do was throw sand on the machines and 80% of the troop and war material movement in the northeast US would come to a halt, thus delaying the US from joining the war. Luckily for us and the rest of the world, the German Nazi soldiers never made it.