"We get in through a long tunnel from the street, protected by blast doors," Bahnhof chief executive Jon Karlung told ZDNet UK.

The tunnel surfaces at 37 Renstiernas Street in Stockholm. It was begun in 1943 and extended during the Cold War to became a civil-defence bunker, stocked with provisions and emergency vehicles. Key staff could survive a nuclear war there, and emerge to rebuild the city.

"The cave was owned by the fire department," Albert France-Lanord, the architect behind the bunker's conversion to a datacentre, told ZDNet UK. "In the '90s, it was used for raves, galleries and an 'underground' theatre scene."

The datacentre is called 'Pionen' ('Peonies' in English), its code name from when it was part of Stockholm's civil-defence system.

"The client did not want to compromise, and gave me quite a lot of freedom," architect France-Lanord said.

Dynamite was required to clear and enlarge the bunker, which had a two-storey concrete structure in one of the side caverns. The floors had to be lowered, to allow for a false floor for cabling.

"The rock walls were treated with lacquer, to show the granite and allow the stone to still breathe, but prevent it from making dust," France-Lanord said. The rock effect on the ceiling is created with sprayed-on concrete — "a usual technique for underground stations", according to France-Lanord. 

The servers are in four caves radiating from the centre of the bunker. This is the view from the meeting room (which has a map of the moon on the floor).

"The hosting centre is built for co-location," Karlung said. "Clients rent rack space and place their own servers in our centre. There is room for about 8,000 servers."

Is the environment good for servers? "It was a happy surprise," France-Lanord said. "We expected humidity, but it's really, really dry in there, and the temperature is very stable."

The plants, water and lighting effects are intended to keep the staff happy. "If you are underground, you don't feel the time go and you get depressed. You need some variation in your surroundings", France-Lanord said.

The metal halide lighting is programmed to give the plants and the 15 full-time staff a natural daily cycle.

Other technologies are emerging in Sweden which could do this, according to France-Lanord. "You can use optical fibre for solar-capture outdoors, and pipe real sunlight indoors," he said. 

The bunker is on Stockholm's electric grid, and has backup power in the form of two diesel engines, originally designed for German submarines. They can provide the 1.5MW of power needed by Pionen's cooling system.

Shown is a rough architectural plan, produced by France-Lanord. Some elements are missing: toilets, for instance. There's a more detailed plan at Arch Daily, which shows toilets (and a room labelled 'lager'), on a second floor above the kitchen.

In three months, Bahnhof has rented out a third of the space. The ISP expects to fill all the space during 2009. "We have actually already started looking for new [datacentre] projects," Karlung said.