A data centre​ is a room or an entire building housing computers, network connection equipment and telecommunications links. Many data centres are built for the exclusive use of just one company, such as Google, but others host – and interconnect – systems belonging to many different users. One of the world’s largest multi-user data centres was once a high-volume printworks. R.R. Donnelley & Sons’ Calumet Plant, on East Cermak Road on the southern edge of the Chicago Loop, printed Time, Life, the Sears Roebuck catalogue and telephone directories. The building is now a data centre used by – among others – telecoms companies, financial exchanges and automated trading firms.

A data centre is no more detached from the brute physical world than a printworks was. Cermak (as it is universally known in the business) is full of stuff. No individual computer server is particularly heavy, but at Cermak there are tens of thousands of them, along with hundreds of miles of cabling, giant generators and transformers, 30,000-gallon tanks of diesel and big power distribution units. So it’s just as well that the building’s floors were originally reinforced to carry heavy printing presses. Cermak is also no less noisy than it would have been in its time as a printworks, although the clanking of machines has been replaced by the steady roar of multitudes of fans and hard disks.

The signals that encode digital information are, ultimately, electromagnetic impulses. The man who showed me around Cermak works for a firm called Steadfast, which rents out space there. He pointed to a cable no thicker than a kettle flex, and told me it was a ten-gigabit fibre-optic connection: it could carry ten billion binary digits every second. The cables are connected up so that the separate, private networks owned by carriers such as Comcast, Verizon and BT can join together to form the seemingly unitary, public internet.

As we walked Cermak’s endless, windowless corridors, with their white walls and anonymous blue doors, we paused (not for too long: Cermak is under continuous video surveillance; lingering looks suspicious) outside Cermak’s ‘Meet-Me Room’, where the crucial cables connect. ‘The backbone of the internet’ goes through Cermak, I was told. The building’s original design helps in this respect. The Calumet Plant had 21 big vertical shafts, through which giant rolls of paper were raised and lowered. Cables now run through these shafts, shortening the paths that signals have to follow between the building’s eight floors.

Everything that goes on in Cermak requires electrical power. Cermak is the second biggest electricity consumer in Illinois, after Chicago’s O’Hare Airport. Its supply must never fail. The building is connected to three different electrical grids, and the tanks of diesel are there to fuel the big generators – of the type, my guide told me, that power cruise liners – that would kick in if all three grids went down.

Nearly all the electricity that flows into Cermak is transformed eventually into heat. Fierce air-conditioning is needed, and data centres used often to be uncomfortably cold. Gradually, their operators have learned better ways of working, for example arranging the computer servers and fans in alternately-facing rows, so that the fans pump heat into a ‘hot aisle’ between the rows, in which the backs of the servers are easily accessible. It’s in those aisles that Cermak’s few maintenance staff mostly need to work. As you walk around, though, you get constant reminders of what it takes to keep Cermak cool: huge pipes carrying chilled water; the occasional blast of very cold air.

The most dramatic episode in the short history of automated trading was the ‘flash crash’ of 6 May 2010, a sudden huge fall – and then almost as rapid a recovery – in prices, which led to a widespread disruption of trading. Cermak was where the crash began. It seems to have been triggered by a set of electromagnetic signals – originally generated by an investment-management firm in Kansas City – encoding a big sell order. At Cermak, the order entered a complex electronic ecosystem of trading algorithms, which would usually have been able to absorb it. For reasons that remain unclear, it didn’t do so on 6 May, so the shock then travelled from Cermak down the fibre-optic cables connecting it to the data centres in New Jersey in which shares are traded, spreading chaos as they arrived.

Human eyes and ears are of limited use in understanding what goes on in a data centre. Even my guide couldn’t enter most of Cermak’s rooms, which are protected by doors with biometric locks. Once you’re inside a room, you get only the most tiny clues as to what’s going on. At least in the rooms housing trading firms’ servers, the computers are generally in locked cages, and those cages are usually tightly packed: renting space in Cermak is pricy, so you want to make the most of it. In one room, though, there was a big cage with only three cabinets of servers, surrounded by expensive, empty floor space. What program could be running on those servers?

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