Watch: I go inside Britain's top secret war bunker, hidden under the streets of Liverpool

You were once required to sign the Official Secrets Act to enter the building; now, it's open for visitors to freely explore. Western Approaches is Britain's top-secret underground bunker, and it's right beneath us here in Liverpool.

Lauren Shacklady, who works at Western Approaches, told LiverpoolWorld: "We only really know what we've heard from the people who used to work here. Very few people actually talked about what they did during the war. It's only those who decided to speak about it at some point, who worked here in this building, that we know so much about the place and how it operated."

With the outbreak of war in 1939, the Royal Navy acquired the newly built Derby House in Exchange Buildings, Liverpool, and work began on a new top secret headquarters, including a two-storey underground bunker. With walls and ceilings of thick, reinforced concrete, the headquarters had a staff of nearly 1,000 Royal Navy and RAF personnel, 80% of them women.

Today, the wartime bunker has been restored to exactly how it used to be and is open to the public as a memorial to those who died to save Britain and the rest of Europe. Visitors can take a step back into history to the original building where the battle was fought and won.

The labyrinth of rooms and offices is a precious time capsule, and although the wartime staff have long gone, a visit to the museum will transport you back to that time and make you ask questions about the people who once worked there.

In May 1945, the last ship was sunk in the Atlantic, and with the surrender of Germany, the Battle of the Atlantic was over. The headquarters remained in operation until August when the Second World War ended. The bunker eventually fell into disuse until it was reopened as a museum in 1993.

• Watch the video above for our full feature and to go under the streets of Liverpool.