Pionen is a former civil defence center built in the White Mountains Södermalm borough of Stockholm, Sweden in 1943 to protect essential government functions.
It was converted into a data center by the Swedish Internet service provider Bahnhof. It opened on 11 September 2008 and Bahnhof continues to use the facility today as of 2019. Because of the facility being buried under the mountain, secured by a 40-centimeter thick door, and only reachable by an entrance tunnel, the data center is capable of withstanding a hydrogen bomb. The Pionen data center is also a colocation centre.
In 2010 WikiLeaks used Pionen's colocation services to store their servers.
It is difficult to visit, only by invitation from Bahnhof, but not impossible.
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