Pionen, Data center in Södermalm, Sweden

Pionen is a data center located roughly 30 meters beneath Stockholm's granite, occupying around 1,100 square meters of underground space. The facility sits behind thick bunker walls and features multiple independent data connections routed through the mountain structure.

Built in 1943 for Stockholm's civil defence, Pionen served as the district control centre for Södermalm and Gamla stan, designed by the Civilförsvarsbyrån. In the mid-1970s it was rebuilt as a forward unit able to resist a nuclear blast wave, with sleeping quarters, water tanks, a power plant and a radio station. Decommissioned after the Cold War, it was rented by Bahnhof in 2007 and opened as a data centre on 11 September 2008.

The interior merges technology with nature through indoor plants, waterfalls, and simulated daylight inspired by 1970s science fiction design. This approach creates an unusual working environment deep underground.

The main entrance is at Renstiernas gata 37, with a second entrance at number 35. Visitors reach the facility through a tunnel and pass a 40-centimeter-thick door. Inside, a vertical emergency shaft rises to a small concrete building with a pyramid roof at Bergsprängargränd on the hill above.

The facility includes a conference room decorated with lunar landscape imagery. The room also features a large saltwater aquarium that adds life to the underground setting.