The squat, signless, concrete structures give nothing away. From the outside they could be any warehouse, anywhere. Their blandness is deceptive--and intentional.

These nondescript buildings that pepper the Loudoun County landscape could be stocked with toilet paper for all the outside world can tell, but they are housing inventory deemed valuable enough to warrant building with reinforced concrete capable of withstanding bomb attacks and AK-47 gunfire.

In these fortresses, agents of the Internet economy are storing what has become the world's most valuable asset: information.

Northern Virginia--especially Fairfax and Loudoun counties--has become a preferred location for what are generically called "data centers," buildings that store the World Wide Web servers and other high-tech equipment used in Internet communication and electronic commerce.

Some firms set up centers to use for themselves: Pacific Gateway Exchange Inc., of California, recently announced that it has leased space in Loudoun Tech Center on Route 7 for its $5 million center; Qwest Communications International Inc., a Denver-based telecom company, plans to locate a facility in Loudoun.

Other companies build centers that act as hosts for outside businesses. For example, Exodus Communications Inc., which has a 100,000-square-foot site near the Route 28 corridor, is a data center and maintains Web servers for companies such as MSNBC, eBay Inc., Lycos Inc. and Reebok International Ltd.

What makes Loudoun attractive to such enterprises is the quantity of fiber-optic cable connections already in the ground.

"One thing about the area is, it has traditionally been the meeting place of networks," said Jay Adelson, chief technical officer of Equinix Inc., a company that serves as a kind of Internet equipment mall for companies such as Internet service providers and telecom firms. The Equinix facility is also near the Route 28 corridor.

The cable already in place allows the company's tenants to connect their networks to the larger Internet backbone that carries voice and data all over the world. And although Equinix could have built a center in Fairfax or elsewhere in Northern Virginia, company officials said Loudoun offered more space at a lower cost.

In addition, Adelson said, Loudoun's Department of Economic Development offered to help speed up the permitting process, which can delay projects such as these because of the number of permits required.

Loudoun development officials said smoothing the way for companies such as Equinix and Exodus makes sense because often these data centers are ideal corporate citizens. "They are phenomenal facilities for us," said Robyn Bailey, the department's marketing manager.

Large capital investments--sometimes as much as $900 a square foot above real estate costs--and small staffs mean that the facilities pay high taxes without adding substantially to crowded roads and schools. Together, Equinix and Exodus have about 40 employees.

Fifteen additional companies are considering locating data centers in Loudoun, Bailey said, although at least a few will probably end up in neighboring counties.

The way she sees it, the best way to attract them is by showing them that the county understands their mission. "The main thing they need is speed," she said, to get their elaborate setups running as quickly as possible.

The Equinix facility is one of the most advanced in the area. Unlike Exodus, Equinix does not sell network access but calls itself an Internet business exchange, acting as a "hotel" for companies that then build their own access. During a rare tour of the Equinix site, Virginia Secretary of Technology Donald W. Upson said recently that he felt as if he were on a movie set.

Equinix spends $1 million to $5 million upfront on security features, depending on the facility.

"A considerable amount of the world's economy is passing over the Internet," Adelson said. A terrorist attack, power failure or anything else that shuts down the high-tech equipment could bring multinational corporations, banks and even governments to a standstill.

To enter the Equinix building without an escort, employees and regular customers' approved representatives must place a palm in a hand geometry reader that has "learned" to recognize the palm. That will get them into the lobby, which, along with the rest of the facility, is bulletproofed and watched by unarmed guards.

To get from the lobby to the area where the Web servers are stored at temperatures slightly cooler than that of a typical office building, they must then enter a "man-trap," a small hallway closed off by a door on each end, and pass another hand reader. The rear door must close before the front door can be opened into the main section of the building.

There is a conference room, shower facilities, even a video arcade for Equinix customers visiting the site to work on equipment or meet with potential clients. Beyond that is a huge room filled with cages.

Inside those cages--each one accessible only by passing yet another hand reader--are the servers and hard drives of companies such as telecom giant MCI WorldCom Inc.

Even if someone could break into a cage, the attacker could not get far, company officials said. Cameras monitor every inch of the Equinix center except the restrooms. The extensive wiring is not stuffed under the floor but is in the open, protected by thick clear plastic, so that no one can cut cables without attracting attention.

The facility also is protected against nonhuman threats. A state-of-the-art fire protection system continually sucks in air and "sniffs" it for any hint of smoke. In case of power failure, the center not only has back-up generators but also enough battery power to provide a day's worth of juice. The facility uses about three times as much electricity as a typical office building its size.

Equinix is spending $20 million to $60 million to build each of its facilities--two others are underway in San Jose and Newark, and it plans to have a total of 12 by the middle of next year. The search for contractors comfortable with installing everything from concrete reinforcements to mammoth cooling systems is so competitive that Adelson does not even like to mention which one his company hires.

"It takes quite a contractor to do this," he said.