You’ll soon be able to delve into a secret world of tunnels used to transport mail hundreds of years back when London’s Postal Museum opens on July 28.

The Mail Rail, which museum representatives claim is the world’s first driverless and electric railway system, came into existence some 100 years back. It was an alternative way to transport mail throughout the city since congested streets and fog were causing delays above ground, according to museum representatives.

When the museum opens this July, visitors will be able to ride through these abandoned tunnels for the first time in the railway’s history.

Hidden from public view, the 6.5-mile-long network of narrow tunnels, which can be just seven feet wide in some areas, sits 70 feet below street level.

Mailbags traveling through #MailRail tunnels only got to their destination if they were correctly labeled. Each bag had a special label with a number relating to a specific station. Image shows #postmen loading bags from conveyor into containers ready to travel in 1935. ⠀ ⠀ ⠀ #blackandwhitephotography A post shared by The Postal Museum (@thepostalmuseum) on Mar 4, 2017 at 4:01am PST

Step back in time as you board the trains modeled after those used in 1987 and take a 15-minute ride along the same tracks once used to hide art treasures of the Tate Gallery and National Portrait Gallery during World War I.

The train ride will be accompanied by an audiovisual show that takes you through the history of the service’s creation and operation as you pass through station platforms underneath of the Mount Pleasant mail center, where the letters and parcels were once collected for delivery.

Once you’ve had a chance to enjoy the subterranean attraction, check out the Postal Museum itself, which will take you through five centuries of the fascinating history of communication in Britain and around the globe.

You’ll find everything from the world’s earliest Christmas card (dating back to 1843) to a plaster cast of Queen Elizabeth II’s head used on stamps in England back in the day as you explore the history of what museum representatives refer to as "the world's first social network."

#DidYouKnow the first Post Office railway tunnels were just 80cm across, about the height of a toddler! #MailRail #openingsoon #underground #hidden #london A post shared by The Postal Museum (@thepostalmuseum) on Mar 14, 2017 at 5:38am PDT

There will also be a play area for children made up of trolleys, pulleys, slides, letter boxes, and a miniature neighborhood of streets and homes they can explore.

Tickets start at £16 (roughly $20).

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