STOCKHOLM (AFP) - On a main street in one of Stockholm's hippest areas, a snow-covered hill topped by a church conceals a nuclear-proof bunker, home to a futuristic data centre holding 8,000 servers -- two of which belong to WikiLeaks.

"All the global fuss is made by these two little boxes," said Jon Karlung, chairman and founder of Bahnhof, one of the companies providing server space to the whistleblowing website.

Karlung is crouched on the floor, pointing to two slim black plastic boxes surrounded by wires. Each blinks with a blue light, indicating that they are active.

The servers are kept in a locked white cabinet along with rows and rows of others in a large room with stone walls -- it has been carved directly into the mountain.

The vault buzzes with the sounds of the servers and the fans needed to cool them down.

The besuited businessman closes the white cabinet door, and continues his guided tour of the data hall, the centre of much attention since WikiLeaks, a client since October, started releasing a slew of secret US embassy cables.

By the chairman's own admission, the data centre is essentially like any other, and WikiLeaks is treated just like any other client Bahnhof provides server services to.

But the place looks like something straight out of a science-fiction or espionage film, reflecting the secretive character of its most talked-about tenant, WikiLeaks' enigmatic leader Julian Assange.

Assange is now sitting in jail in London pending a hearing on extradition to Sweden, where he is wanted for questioning over sexual assault allegations. Until Interpol sent out a notice for his arrest, even his exact date of birth had been kept secret.

A visitor enters the data hall through sliding glass doors, engulfed in the steam caused by the evacuated heat of the servers, on the side of Vita Berget (The White Mountain) in a trendy corner of the Soedermalm borough of Stockholm.

Once inside, the heat and humidity are stifling as a slightly slanted ramp leads into the hall itself, the wall flanked by almost tropical plants that would never survive the chilly temperatures outside.

Code-named "pionen" (the peony in Swedish) the bunker was first designed in the mid-1940s, then refurbished as a nuclear-attack proof civilian defence shelter at the height of the Cold War.

The large hall was used for different purposes -- at one point in the 1990s it was an exhibition space, until being taken over a few years ago by Bahnhof, a large company with four other -- far more ordinary -- data centres. The company is also an Internet service provider.

The bunker provides extra security for WikiLeaks, Karlung jokes, but the real threat is not really a physical one.

"We are very well protected for physical attacks, but that is not going to happen. The real threat is maybe legal and probably cyber attacks," he says.

In an office next to the server room, he proudly points to a screen showing a graph of the traffic on the WikiLeaks servers.

"Up to now, no attack has hit us directly. We have seen effects from other attacks, but no attack on this facility or the services they have here," he says.

WikiLeaks, Karlung explains, also has servers elsewhere.

"They don't have all their eggs in the same basket."

When asked about the political turmoil unleashed by cablegate, Karlung says his clients can use their servers for what they want, as long as they do not break Swedish laws.

"The only thing that would jeopardise their servers here is if they had illegal material ... They must pay their bills, their material must be legal in Sweden."

Hosting a server, he says is "just like the mail service."

Asking him what his clients use their servers for is "like asking the mailman if he opens the mail," he says.