We now know what Facebook is up to in Sub-Zero – the mini data center it's building right beside its 330,000-square-foot Prineville, Oregon, facility: It's reinventing the way it does emergency backup.

The plan is to use the building to house a brand-new type of low-power deep-storage device that Facebook engineers will cook up over the next six to nine months. They're designing a hard-disk storage server that powers off when it's not in use, says Tom Furlong, vice president of site operations at Facebook. "It's going to sit in a dedicated building that is optimized to support this device that we don't need to access very often."

What will this building be like? Boxy and quiet, with rows of low-powered machines clicking on and off, says Furlong.

The company disclosed the existence of the the new building in recently filed planning permits. In the permits, Facebook called the building Sub-Zero, though Furlong says he's not using that name in public (the Sub-Zero trademark is already taken by a line of high-end refrigerators, thank you very much).

In addition to the 62,000-square-foot Sub-Zero building, Facebook is in the process of building a second 330,000-square-foot data center in Prineville. Just down the road from the Facebook facility, Apple is constructing its own dataplex, this one with 500,000 square feet of server-rack space.

Sub-Zero will do the kind of large-scale deep archiving that some companies still achieve with tape backup.

Facebook doesn't use tape backups – they're too kludgy and expensive for the social media company. But it does save two backup copies of all of its data: one that is there to be used whenever there's a problem on the live server, and a second copy for emergency use only.

"You have one that is pretty readily accessible, and you have one that does not need to be accessed except in the event of a true restore," says Furlong.

So Facebook wants to power down these secondary backup storage drives until they're needed.

They're only in the early stages of design now – the team has about six to nine months to work out how it will build the Sub-Zero storage servers and tweak the software on its network to work with them.

Apple's Tactical Data Center. Photo: Garrett Fisher/Wired

They hope to seriously cut power consumption with Sub-Zero. Right now a rack of Facebook servers burns about 4.5 kilowatts. In the Sub-Zero data center, the goal is to drop this to around 1.5 kW.

Facebook wants to add a second Sub-Zero-type storage facility next to its Forest City, North Carolina, data center, Furlong says.

Facebook isn't the only company adding smaller appendixes to its massive data centers. Apple recently tacked on a 23,000-square-foot building next to its Maiden, North Carolina, building. Nobody but Apple knows what the company is doing in what it calls its "tactical data center." Some think that it might be a place for partners to come by and plug into Apple's networks without being tainted by their mind-blowing computing secrets.

Could the tactical data center be Apple's own version of a cold-storage facility? Perhaps. Facebook has opened up its data center and server designs but Apple is notoriously secretive about its own operations.

Furlong hasn't heard of anyone else building a cold storage facility like Facebook's, but that doesn't mean it isn't happening. "I think we're pretty smart," he says, "but I don't necessarily give us a monopoly on having identified this as a problem."