A data center has three problems and the building is only one of them.
It has to be cool, because every watt that goes into a server comes back out as heat within a second.
It has to be secure, since the machines hold things worth taking.
And it has to be reliably powered, because a few seconds without electricity costs more than the building did.
Most operators solve those separately, with chillers, fences and generators bolted onto a shed.
A hole in a mountain solves two of the three before anyone builds anything.
Why a rock face is a cooling asset
Granite at depth sits at a steady temperature all year, because the seasons never reach that far down.
Above ground a cooling system fights whatever the weather is doing. In August it works against warm air, in January it can barely run at all without controls to stop it overcooling.
Underground the outside condition stops changing, which makes the whole plant a simpler machine.
Rock also has enormous thermal mass, so a surge of heat is absorbed by the surrounding stone rather than immediately raising the air temperature in the hall.
The catch is that rock is a poor conductor and does not carry heat far, so the stone near the hall warms up over time and stops helping.
Which is why a buried hall still needs real cooling plant, just a less frantic one.
The shelter that was already there
The space was cut in 1943, as a civil defense shelter under a hill in the middle of the Swedish capital.
That is the useful inheritance. Somebody had already driven an access tunnel through the bedrock, already blasted a cavern, and already built a door thick enough to be measured in feet.
None of that could be afforded today. Tunneling into granite in a city center is a permitting exercise as much as an engineering one.
What the shelter did not have was room, or a floor, or anything a server hall needs.
So the conversion was not a fit out. It was a second excavation.
More than 4,000 cubic yards of solid rock came out to make the space usable, which is roughly the volume of a suburban house, removed one blast at a time through a single tunnel.
Two submarine engines and a saltwater tank
The hall sits about 100 feet below street level under granite, with a floor area near 12,000 square feet.
Cooling runs at 1.5 megawatts, enough for several hundred racks, and backup power comes from two diesel engines originally built for German submarines, each rated at the same 1.5 megawatts.
The redesign ran through 2007 and 2008, by a Stockholm architecture practice, and the facility opened in September of 2008.
The design brief was unusual and the operator has been open about it. Rather than optimizing only for machines, the space was built around the people who work in it.
So there are greenhouses, fountains, simulated daylight and a saltwater fish tank, underground, in a bunker.
The stated reason is that designers of these places forget the humans who have to sit inside them.
What a bunker is not good at
Depth buys a lot and it costs things that rarely get mentioned.
Expansion is the first. A shed can have another shed built beside it. A cavern can only grow by blasting again, with the live facility running next door.
Connectivity is the second, since fiber has to come down the same tunnel everybody else uses, which concentrates a risk that a surface building can spread across several routes.
Rejecting heat is the third and the largest. All that energy still has to leave, and a cavern has no roof to put dry coolers on, in the way that a tall timber building or a printed structure trades one constraint for a different one.
The architects themselves described the result as a contrast between raw stone and a designed interior, calling it a bunker fit for a film, which is a fair description.
Why nobody copies it
The obvious question is why every data center is not in a mountain, and the answer is arithmetic.
Blasting rock costs many times what a steel frame costs per square foot, and the cooling advantage is worth a few percent on the power bill rather than a multiple.
This project worked because the expensive part was paid for during a war, by a government preparing for something else entirely, eighty years before anybody needed a server.
That is not a business model. It is an inheritance, and there are only so many of them.
Sweden has hundreds of these caverns from the same period, and a handful have found second lives as archives, stores and server halls, as the trade coverage of the site notes.
The solid rock is not why this place exists.
The hole in it is.
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