Pionen White Mountains in Stockholm, Sweden is a unique data center located in a Cold War era nuclear shelter, deep in the bedrock, right underneath Stockholm city. It was designed to be able to withstand a near hit by a hydrogen bomb and is utilised by Sweden's largest ISP, Bahnhof, it is well protected and well connected.

Pionen is one the best-connected places in Northern Europe. It offers triple redundancy Internet backbone access.

The network has full redundancy with both fiber optics and extra copper lines with multiple different physical ways into the mountain.

Backup power supply is ensured by two Maybach MTU diesel engines that were originally designed for submarines. They are capable of producing 1.5 Megawatts of power. Just for fun, a warning system (sound horns) from the original German submarine has been installed.

The cooling is handled by "Baltimore Aircoil fans producing a cooling effect of 1.5 megawatts". This is enough to cool several hundred rack-mounted servers at a time.

Boasting 11,950 sq ft of space the data center houses a Network Operations Center (NOC) for all of Bahnhof's operations.

Pionen's network comprises multiple main production servers, backup servers, as well as administrative, spare and dedicated servers.