You've got mail!

NORTHBOROUGH -- Six million people know the experience: You order a DVD from the Netflix website and a day or two later it appears in a red paper envelope. You watch the movie, mail it back, and soon another disc arrives. This cycle happens in America 20 times every second and 1.6 million times every day, making Netflix, the movie-rental company, a veritable postal service within the postal service.

How does the right movie get to the right person on the right day with greater than 99 percent accuracy? Only a privileged few people know. Netflix guards its patented disc-tracking technology like the Coca-Cola Co. guards its famous formula.

"That's the secret sauce," Netflix communications director Steve Swasey says. "The key to the kingdom."

But -- shhh! -- clues to the mystery exist here, where Netflix opened a regional distribution center in 2002. Roughly the size and shape of an airplane hangar, the unlabeled warehouse employs a small army of 55 red-shirted Netflix workers who process 60,000 DVD orders per day from southern New England. These people -- and those in 45 other regional facilities across America who inspect, sort, and ship every DVD -- are the Christmas elves of the Netflix empire. Or, to use a movie metaphor, they are the Oompa Loompas of Willy Wonka's factory.

Netflix purposefully disguises its warehouses. The exterior of the 40,000-square-foot Northborough facility contains not a single Netflix logo. Employees must access the building through a remote side door and sign confidentiality pledges under penalty of termination. DVDs arrive and leave in unmarked Ryder trucks.

Why the Pentagon-level secrecy? Two reasons, Swasey says. First, Netflix subscribers might want to return their DVDs by hand rather than by mail, which could disrupt the sensitive disc-tracking system.

But second and most important, Netflix wants to protect its technological secrets against snooping rivals. Swasey is careful not to use the B-word, but Netflix's largest competitor, Blockbuster, introduced an online-rental program in 2004, and Netflix sued its rival last year for patent infringement.

In order to tour the facility, Netflix required the Globe to withhold the building's address and denied access to several critical steps in the process. Still, the tour offered a peek behind the curtain of one of America's most secretive companies -- and one that operates in stealth in Boston's backyard.

The first thing you notice inside the Netflix warehouse are the red T-shirts. They're on every employee. And they're practically the only bright-colored items inside the drab industrial facility, decorated only with a sparse and random collection of movie posters: "Starsky and Hutch," "Monster-in-Law," and "TMNT," the Teenage Mutant Ninja Turtles movie.

The warehouse -- a football field-size room full of whirring machines and silent workers -- contains two human assembly lines: one for incoming DVDs and one for outgoing DVDs. Incoming discs are ripped from their envelopes by employees called associates, inspected for damage, discarded if broken or repackaged in paper sleeves if not, and sent to storage. Northborough facility employees process 750 discs per hour per person from 5-to-9 a.m. During this all-important morning shift, they inspect, filter, and file 60,000 discs -- and leave behind mountains of recyclable red envelopes.

What happens next is anyone's guess. Netflix won't explain how the bar codes on each DVD sleeve work -- except to say that they're the company's magic ingredient. Where are DVDs stored? How do machines locate a single disc from a sea of options? How many DVDs get lost in the shuffle?

"Sorry," Swasey says. "Confidential."

More than 95 percent of Netflix movies are rented at least once every three months -- and most get circulated far more often than that. "Netflix DVDs are like electric currents," Swasey says. "They're always in motion."

To keep the current flowing, the outgoing assembly line operates as rapidly as the incoming line -- but in reverse. In trays of 300, DVDs are taken out of storage. An employee drops the discs into a machine that slots 3,000 per hour into barcoded envelopes. Then, the DVDs are marked with addresses, sorted by ZIP code, and driven in the Ryder trucks to the post office. Ninety percent of DVDs reach a subscriber's mailbox within a day after the order.

The Northborough warehouse processes less than 4 percent of Netflix's total workload. Other regional "hubs" in Portland, Maine, and Long Island, N.Y., handle Netflix customers in northern New England and the New York area, respectively. And some obscure titles are mailed to southern New England from warehouses outside the region.

Since 2002, Netflix has opened eight to 10 hubs per year in cities and towns across America. The goal is to standardize the time it takes for DVDs to arrive -- whether you live in New York City or Fargo, N.D. Although the company recently opened warehouses in Anchorage and Honolulu, Swasey says Netflix has no plans to go international. "We're trying to build strength in America," he says.

Every Netflix distribution center is identical. "This is very much a cookie-cutter operation," Swasey says. "Sometimes I have to think, 'Where am I right now?' That's how much all the hubs look the same."

Again, he stresses, the goal of Netflix is efficiency and speed. If the DVD distribution system operates rapidly in Los Angeles, Netflix believes, it will work equally fast in, say, Lexington, Ky.

The need for speed is clear: Changes in the movie-delivery business are getting more intense. While Blockbuster's online-ordering program boasts only 3.6 million subscribers to Netflix's 6.7 million, Blockbuster added 600,000 subscribers during the April-June quarter while Netflix lost 55,000 members. In response, Netflix cut prices to match Blockbuster's.

On the horizon is an even more pressing development -- the growth of services that allow viewers to rent or download movies directly on their computer, with no need for envelopes, red or otherwise. Netflix recently introduced a program that makes 40,000 movies available for downloading.

That's just a fraction of the total titles Netflix can zip to your mailbox. Whereas Blockbuster makes more than 80 percent of its profit from new releases, Swasey says, Netflix customers order non-new releases 70 percent of the time. Toward the end of the tour, Swasey dips his hand into a random pile of recently ordered DVDs. He pulls up the teen television show "Dawson's Creek," the German drama "Mephisto," the science-fiction series "Battlestar Galactica," and the 1960 French release "Eyes Without a Face."

"It really shows the breadth and depth of American tastes in movies," he says.

Back at the warehouse entrance, Swasey greets one of the Northborough facility's managers, James Gage. Like all Netflix employees, Gage can tell friends where he works -- but not the facility's exact address. Asked what friends normally ask about his job, Gage, a Worcester native, responds: "How do you guys do it? How do you get all the DVDs in the right place?"

But before he can even start to answer that question, Swasey flashes him a stern glance. It can mean only one thing: Confidential.

Robbie Brown can be reached at jbrown@globe.com.