Beneath the streets of Tallinn, Estonia lies a number of passages that ran under the city's original defensive bastions built in 1630's. It's possible to visit these passages as part of tour organised at the Kiek in de Kok (translation: Peep into the Kitchen) tower.
Following an audio-visual presentation on the history of Tallinn and its defences, you're taken down into the tunnels themselves. The tour is split into 2 parts with the first detailing the tunnels usage through the ages, heading into the past with depictions of them as a haven for tramps and punks, shelters during the soviet era and bomb shelters during World War 2 before detailing the tunnels original construction.
The second part of the tour looks to the future, specifically to the year 2219. As you travel on a sideways train a presentation plays out detailing the challenges facing the planet over the next few years. The journey ends in a small exhibition with everyday items of today display as museum pieces.
For more information on the tunnels, I suggest having a look at here.