Inside the history of one of London’s most iconic landmarks: Piccadilly Circus ABC News were granted an exclusive look inside the "beating heart of London."

LONDON -- Very few -- if any -- of the hundreds of thousands of Londoners who take the London Underground through Piccadilly Circus every day think about what’s beneath their feet. But behind the locked doors is a warren of vacant passages and platforms.

Siddy Holloway is the curator of Hidden London tours for the London Transport Museum. She told ABC News that the most common reaction to this secret side of the capital is "awe."

The original, defunct Piccadilly Circus, which is the subject of this Hidden London tour, was built in 1906. However, due to the huge volume of passengers that quickly descended on the station, a modern one had to be built in 1928. Now, the iconic tube stop accommodates 40 million passengers every year.

“It’s just beyond the reach of what you can see as a commuter,” Holloway said.

The escalator, an invention that would revolutionize commuter transport in the early 20th century, changed the underground train station forever.

“Many of these first underground stations were all built with lifts, but when the escalator comes into existence in 1911, that means they all want to be fitted with escalators,” Holloway said. “This means you have to completely remodel all stations, and so there’s actually a lot of disused station part. I like to call them forgotten pockets of history.”

The storied station has been a part of many historical London events, including its role in the Blitz during World War II and the rapid expansion of urban transport in the early 1900s. The station’s redesign was called the "Heart of London" because of its classic art deco style during the height of the British Empire.

“People are always very fascinated by wartime in the Second World War and how the tube was used for that," Holloway said. "Piccadilly Circus was one of the busiest tube shelters in London at the time, with about 7,000 people sheltering here every single night.”

In addition to tours of Hidden London, there will be an exhibition opening on Oct. 11 in the London Transport Museum. An accompanying book, "The Hidden London," goes on sale Oct 10.