"The cloud." The phrase is ubiquitous today, a buzzword, a shibboleth of the technorati. In some senses, the image of a cloud is a fitting metaphor for an increasingly common feature of modern computing--the storage of data in a location other than the device being used to access that data. It helps communicate, simply, the idea that our data floats around us, no matter where we go.

But in another sense, the image of the cloud is sorely lacking as a metaphor. It remains a little, so to speak, nebulous. Using the cloud is a light, mobile, free-wheeling experience. But the cloud itself--the machinery that undergirds it--is anything but light. As these surprisingly artful images from Interxion, a European data center company, illustrate well, the cloud is a heavy, heavy thing.

An important thing to remember when constructing a data center is that the cloud is also a very hot thing, too, one of the reasons Google just opened a $273 million server hall in Hamina, Finland. "In a room that's, for example, 1,000 meters squared, you can fit an awful lot of computers into that space," says Graeme Creasey, Interxion's Director of UK Operations. "That starts to generate an awful lot of heat." PIctured here, part of a chilled water cooling system in an Amsterdam facility.

In a London facility, servers are kept together in units circulating cold air. The yellow strips you see are cables offering connectivity to the outside world; they're kept in neat rows so as not to impede circulation.

This red tank, in the Amsterdam facility, allows hot and cold water to be mixed, as part of the building's heat exchange system.

Pictured here, the transformer switch room, which funnels power from the outside world onto the data center floors. "I could go on for hours on this," said Creasey, growing excited. Switchboards, breakers, and busbars running overhead direct the electricity.

"Here's a saucy image!" says Creasey. Redundancy, he points out, is key to successful cloud services--if the outside power supply should falter, you still want your customers to be able to access their data. In London, for instance, 60% of Interxion's customers are in the financial sector; even a small amount of downtime could translate into millions of dollars of losses. So while Interxion hasn't had to use the backup generator pictured here just yet, it's nonetheless a crucial piece of equipment (along with its five brothers).

On the roof of the London facility are fans allowing hot air out, and cool air in. In the background can be seen the Gherkin, a landmark of London's financial district. Wouldn't it be cheaper to house servers somewhere other than London? Not so; for those financial customers in particular, many of whom deal with split-second trading, physical proximity to the data center can mean faster trades and greater profits.

The cloud comes in all the colors of sunset; here, bright red cabling.

A corridor running between server rooms (in the Amsterdam facility) features overhead cable trays and, of course, fire extinguishers.

With multiple companies housing their servers in one location, security becomes paramount. The so-called "man traps" featured here ensure that people entering the facility are who they say they are. Entering the facility requires swiping a keycard and having one's fingerprints read. Man traps behave like something out of Get Smart; the front panel opens, the customer steps in, and the panel closes again, at which point another panel opens to allow entrance. The cloud, in other words, is also something of a fortress.

Sunset atop the Amsterdam facility, with heat exchange units and lightning rods. Of the cloud, says Creasey, "It is light and fluffy, very much so, for the end user--the boy or girl walking around with iPad, accessing maps"; on the back end, though, it's a very different story. Asked what all the equipment in one facility weighed, Creasey paused to do a quick calculation. "200 tons, give or take," he said. "Plus a building on top."