It began with an Xbox game.On a recent rainy evening in Brooklyn, I was at a friend’s house playing (a bit sheepishly, given my incipient middle age) Call of Duty: World at War. Scrolling through the game’s menus, I noticed a screen for Xbox Live, which allows you to play against remote users via broadband. The number of Call of Duty players online at that moment? More than 66,000.Walking home, I ruminated on the number. Sixty-six thousand is the population of a small city — Muncie, Ind., for one. Who and where was this invisible metropolis? What infrastructure was needed to create this city of ether?We have an almost inimical incuriosity when it comes to infrastructure. It tends to feature in our thoughts only when it’s not working. The Google search results that are returned in 0.15 seconds were once a stirring novelty but soon became just another assumption in our lives, like the air we breathe. Yet whose day would proceed smoothly without the computing infrastructure that increasingly makes it possible to navigate the world and our relationships within it?Much of the daily material of our lives is now dematerialized and outsourced to a far-flung, unseen network. The stack of letters becomes the e-mail database on the computer, which gives way to Hotmail or Gmail. The clipping sent to a friend becomes the attached PDF file, which becomes a set of shared bookmarks, hosted offsite. The photos in a box are replaced by JPEGs on a hard drive, then a hosted sharing service like Snapfish. The tilting CD tower gives way to the MP3-laden hard drive which itself yields to a service like Pandora, music that is always “there,” waiting to be heard.But where is “there,” and what does it look like?“There” is nowadays likely to be increasingly large, powerful, energy-intensive, always-on and essentially out-of-sight data centers. These centers run enormously scaled software applications with millions of users. To appreciate the scope of this phenomenon, and its crushing demands on storage capacity, let me sketch just the iceberg’s tip of one average individual digital presence: my own. I have photos on Flickr (which is owned by Yahoo, so they reside in a Yahoo data center, probably the one in Wenatchee, Wash.); the Wikipedia entry about me dwells on a database in Tampa, Fla.; the video on YouTube of a talk I delivered at Google’s headquarters might dwell in any one of Google’s data centers, from The Dalles in Oregon to Lenoir, N.C.; my LinkedIn profile most likely sits in an Equinix-run data center in Elk Grove Village, Ill.; and my blog lives at Modwest’s headquarters in Missoula, Mont. If one of these sites happened to be down, I might have Twittered a complaint, my tweet paying a virtual visit to (most likely) NTT America’s data center in Sterling, Va. And in each of these cases, there would be at least one mirror data center somewhere else — the built-environment equivalent of an external hard drive, backing things up.Small wonder that this vast, dispersed network of interdependent data systems has lately come to be referred to by an appropriately atmospheric — and vaporous — metaphor: the cloud. Trying to chart the cloud’s geography can be daunting, a task that is further complicated by security concerns. “It’s like ‘Fight Club,’ ” says Rich Miller, whose Web site, Data Center Knowledge, tracks the industry. “The first rule of data centers is: Don’t talk about data centers.”Yet as data centers increasingly become the nerve centers of business and society — even the storehouses of our fleeting cultural memory (that dancing cockatoo on YouTube!) — the demand for bigger and better ones increases: there is a growing need to produce the most computing power per square foot at the lowest possible cost in energy and resources. All of which is bringing a new level of attention, and challenges, to a once rather hidden phenomenon. Call it the architecture of search: the tens of thousands of square feet of machinery, humming away 24/7, 365 days a year — often built on, say, a former bean field — that lie behind your Internet queries.