Inside New York City's most secret basement

Most visitors to New York's Grand Central Terminal probably have no idea they tread over one of the most secret basement vaults in the history of the city.

Under 10 stories of subterranean marble and upholstery shops, power plants and other substructures lies a giant room rich in history and unknown to most. At certain times in the past, anyone who mistakenly found themselves in the elevator to this room risked being shot at the bottom in order to keep the vault's secret safe.

What exactly did this room contain to warrant such protection?

At 22,000 square feet -- almost as large as the station concourse above -- the chamber housed nine huge "rotary converters" weighing 15 tons each. These converters transformed over 11,000 volts of alternating current in the direct current needed to power the trains above.

The converters were the beating heart of a system that powered the Northeastern rail system -- over 2000 miles of heavily used tracks.

When the rotary converters were installed in 1913, the purpose of having them below ground was to allow for more profitable development of skyscrapers on the surface. However, the protective nature of the underground chamber came in handy by the outbreak of World War II.

The northeastern railways were vital in moving troops and equipment to the eastern ports. If the converters under Grand Central Terminal were damaged or destroyed, the US war effort could have been crippled.

Indeed, the room was targeted by Nazi spies. Luckily for the Allied war effort, the saboteurs were spotted coming ashore from their U-Boats by a Coast Guard watchman. The group was intercepted by the FBI before they could stage their attack on New York's most secret basement.

Also on AOL:
Woman treads water for 13 hours in Gulf of Mexico
Hundreds of fish to fall from the sky into Utah lakes
Ship wreck explorers solve decades old plane wreck mystery