Mount Weather is a top-security underground installation an hour's drive from Washington DC. It has its own leaders, police, fire department - and laws. A cold war relic, it has been given a new lease of life since 9/11. And no one who's been inside has ever talked. Tom Vanderbilt reports

'Actually, you may want to just put those down a minute," Tim Brown is telling me, as I peer through binoculars at a cluster of buildings and antennae on a distant ridge. "The locals might get a bit nervous." A Ford F-150 cruises by, and the two men inside regard us casually as they pass.

We are sitting, hazards blinking, in Brown's BMW on a rural road in Virginia's Facquier County, a horsey enclave an hour west of Washington DC. The object of our attention is Mount Weather, officially the Emergency Operations Centre of the Federal Emergency Management Authority (Fema); and, less officially, a massive underground complex originally built to house governmental officials in the event of a full-scale nuclear exchange. Today, as the Bush administration wages its war on terror, Mount Weather is believed to house a "shadow government" made up of senior Washington officials on temporary assignment.

Following the collapse of the USSR, Mount Weather seemed like an expensive cold-war relic. Then came September 11. News reports noted that "top leaders of Congress were taken to the safety of a secure government facility 75 miles west of Washington"; another reported "a traffic jam of limos carrying Washington and government license plates." As the phrase "undisclosed location" entered the vernacular, Mount Weather, and a handful of similar installations, flickered back to life. Just two months ago, a disaster-simulation exercise called Forward Challenge '06 sent thousands of federal workers to Mount Weather and other sites.

Mount Weather is not hard to find. From the White House, we take Route 66 west until it meets Highway 50. Fifty miles later, we turn off on Route 601, a small two-lane rural feeder that snakes up a ridge. That road seems to be going nowhere until suddenly, at the crest, we come into a clearing, bounded by two lines of tall, shiny, razor-wired fencing, marked with faded signs that say: "US Property. No Trespassing." Behind sits a grouping of white aluminium sheds and a few cars.

We have arrived at the edge of the known republic. What lies beyond is obscured by Appalachian scrub and the inky black of government classification. No one has ever been allowed to tour the underground complex at Mount Weather and tell of what they saw. Occupying 500 acres of Blue Ridge real estate, it functions like a rump principality, with its own leaders, its own police and fire departments, and its own set of laws.

Mount Weather is more easily viewed from outer space than down the block. Earlier in the afternoon, I had been looking at grainy 1m-resolution aerial images of Mount Weather assembled by Brown, a national security researcher and aerial imagery expert. He pointed to small notches on the side of a hill (tunnel entrances), helipads, and a series of "military-style above-ground soft support housing". The mountain straddles the two entrances, he noted. "It's something like 200ft of shelter on top of you at the highest point."

Just driving round the perimeter of Mount Weather, you can see the traces of recent work. "See how they've obscured this," he says, pointing to the black sheeting threaded through a length of fence. "You used to be able to see the helipad through that fence." He gestures towards the new entrance. "Look at the truck barriers. When you turned, there'd be no time to build up speed. They got smart."

The changes to its exterior landscape - not to mention the gossip among local residents - are just one sign that that something very important has been going on at Mount Weather, a level of activity not seen here since the days when Eisenhower and his advisers trooped out here during drills. For some, this is a sign of prudent planning in a world where the security calculus has been for ever altered; for others, it is the symbol of an administration with a predilection towards exercising power in secret. As we pull away from Mount Weather, Brown says, "I wouldn't want to be driving a rental truck and have it break down in front of the gate."

Mount Weather first caught the American imagination on December 1 1974, when a Dulles-bound TransWorld Airlines 727, struggling through heavy rains and 50mph winds, crashed into the top of the mountain, less than a mile and a half from the site. The crash briefly severed the underground line linking to the Emergency Broadcast System, and teletype machines in news offices across the country began spitting out garbled transmissions.

The story might have died there. With Vietnam and Watergate in the air, however, the words "secret government facility" did not exactly induce a frisson of patriotic glee. The Progressive, in 1976, published an article, entitled The Mysterious Mountain, which said Mount Weather, a place little known even to Congress, was home not only to a replica mini-government, but to files on at least 100,000 Americans. In 1991, Time published the fullest exposé, describing (based on conversations with retired engineers) a sprawling underground complex bristling with mainframe computers, air circulation pumps, and a television/radio studio for post-nuclear presidential broadcasts.

What information has emerged about Mount Weather has always been rather sketchy. At some point in the 1950s, however, it seems that a drilling experiment into the mountain's rugged foundations of Precambrian basalt was turned into an exercise in underground city building, with the army corps hollowing out of the "hard and tight" rock a complex of tunnels and rooms with roofs reinforced by iron bolts.

The base formed part of a "federal relocation arc", an archipelago of hardened underground facilities, each linked by a dedicated communications system and equipped with amenities ranging from showers to wash off nuclear fallout to filtration systems capable of sucking air clean down to the micron level. The sites, staffed by "molies", were spartan steel-and-concrete expanses, subterranean seats of power: the president could repair to Mount Weather; Congress had its secret bunker under the Greenbrier Hotel in West Virginia; the Federal Reserve had a bunker in Culpeper, Virginia; the Pentagon was given a rocky redoubt called Site R in the mountains of southwestern Pennsylvania; while the nation's air defences were run out of Norad's (North American Aerospace Defence Command) Cheyenne Mountain facility. "The nuclear age has dictated that these men carry out their responsibilities inside a solid granite mountain," wrote the defence command.

Mount Weather's secrecy was never absolute. In the 1957 novel Seven Days in May, the authors referred to a shadowy facility called Mount Thunder, all but revealing its location. Driving around those Blue Ridge byways today, a curious mixture of secrecy and openness still prevails. On Route 601, an Adopt-a-Highway sign is sponsored by employees of the Mount Weather Emergency Operations Centre. But pull off toward the entrance of that facility, and things get a bit strange. Looking for the home of a local resident, I hail an exiting Mount Weather employee. As we begin to chat, cars side by side, I suddenly hear a strange, siren-like sound and notice that a black SUV has loomed into my rear-view. The occupant, wearing sunglasses, hastily points me in the right direction.

This contradictory world of sunshine and shadow is at one with the parallel nature of the facility itself. On the one hand, it is, as Fema describes it, "a hub of emergency response activity providing Fema and other government agencies space for offices, training, conferencing, operations and storage". Less discussed is Mount Weather's obliquely assumed status as one of the key "undisclosed locations" of the Bush administration. "Look, there are two Mount Weathers - there's the Fema one and the Mount Weather one," says John Weisman, a writer of military and spy thrillers and a neighbour of the facility. "I wouldn't be at all surprised if [the vice-president, Dick] Cheney had been here before, and if [the secretary of defence, Donald] Rumsfeld had been here before, because they were part of some hugely sensitive stuff that was going on in the 1980s."

Weisman is referring to a series of classified programmes, described by the journalist James Mann in The Rise of the Vulcans, in which Cheney and Rumsfeld were said to be "leading figures". According to Mann, the resurgence of tensions with the Soviet Union during the Reagan administration lent new urgency to "continuity of government" programmes. With a secret executive order, and an "action officer" in the form of Oliver North, top officials pondered such constitutional quandaries as whether it would be necessary to reconstitute Congress following a nuclear attack (the answer was no).

On September 11 2001, Mann writes, the long-dormant plan was activated, and any number of top officials - possibly including Cheney himself - were shuttled to Mount Weather.

Residents on the mountain did not need to read the newspapers to discern that something was going on there. Joe Davitt, a retired civil servant who lives in a small A-frame house a mile or so away, told me that on September 11 2001 his wife was returning home from Florida. At the bottom of the hill, he says, she was stopped by state troopers, who asked for identification. At the facility itself, he says, "The Mount Weather guards were not only armed, they had their guns in firing position." John Staelin, a member of the Clarke County Board of Supervisors, says that on September 11, the county's 911 line received a call from an agitated local woman. "She said, 'I wouldn't have believed it if I hadn't seen it with my own eyes, but the whole mountain opened up and Air Force One flew in and it closed right up. I wouldn't have believed it if I hadn't seen it with my own eyes.' So they said, 'Yes, ma'am.' "

Whatever else, Mount Weather makes for an interesting neighbour. "My God," says Davitt, as we sit on his back porch, "they put a plough up there at the first forecast of snow. They've always been good at keeping the road ploughed."

"We call our house ground zero," says Weisman. "This mountain has its interesting moments, between the helicopter flights and the people coming and going." Where for years "Mount Weather was nothing but a sleepy little byway", Weisman complains that the post-9/11 security adjustments have only served to draw attention to the facility. "It now says, 'Boy, am I important!' "

The local people are, by and large, perfectly happy to talk about Mount Weather. Sometimes, however, a veil of secrecy descends. When I asked about Mount Weather at the Daily Grind coffee shop in nearby Berryville, a woman smiled nervously and told me one woman she knew saw "missiles" being taken there. I was forwarded an email from a mountain resident (with the .mil domain that suggests a military background) that contained complaints about late-night helicopter flights, as well as recent episodes of nocturnal machine-gun fire and even a "massive explosion" that had shaken the house. My email seeking further comment received an immediate, terse response demanding that the sender not be associated with the story.

Inquiries to Fema yield little more light. "There's been a general upgrade of security at all federal installations around the country, and Mount Weather is one of them," says spokesman Don Jacks. "I answered your question in a very general way. We're not going to talk about Mount Weather, period. It's not that I can't, we just don't."

A request to talk to Reynolds Hoover, the director of Fema's Office of National Security Coordination, dies on the vine. And forget about James Looney, Fema chief at Mount Weather. "To talk to Mr Looney you would have to talk about Mount Weather," Jacks reminds me. "And we don't talk about Mount Weather."

One afternoon, I went to have lunch with Jim Wink at the Horseshoe Curve, a saloon tucked away near the hamlet of Pine Grove. It has been the unofficial canteen of Mount Weather for as long as anyone can remember. "I've seen Seabees [members of the US Navy Construction Battalions] come out of the tunnels at the end of the day and come down to the bar for a few beers," says Weisman. A Comanche pickup in the parking lot has a bumper sticker that says Terrorist Hunting Permit.

"I checked you out last night," Wink says by way of introduction. "So did Ray." He's talking about Ray Derby, a former Mount Weather employee whom I had visited the night before, who has suddenly appeared today. Wink, an Irish-blooded South Philadelphian with a tight smile and a steely, penetrating stare, does not seem like a man of whom you would like to run afoul. A retired counterterrorism expert with stints in the CIA, the Secret Service and any number of other agencies, he seems to have been in every place in the world at the most politically sensitive time. He was one of the last several hundred US personnel in Vietnam in April 1975, until he heard the song White Christmas - a coded message to get out of the country.

His office is filled with memorabilia culled from the more occluded arenas of US foreign policy; there is a plaque signed by the team tracking the Shining Path leader, Abimael Guzman, in Peru; a collection of Wink's identity cards from various intelligences agencies around the world (he's wearing sunglasses in most of them); and, among other souvenirs, a photograph of the slain drug lord Pablo Escobar. "There's your richest man in the world," he says, handing me a snapshot of a bloated, blank-eyed corpse. "He did not die a good death."

There's a Vets for North sticker on one wall, and, on another, one that says: "Even My Dog is Conservative."

Wink came to Mount Weather in the 1980s. "I needed a training facility and they offered a great deal up here."

When he came with the Secret Service one day to the Curve for a beer, he met his future wife, Tracee, whose grandfather had owned the bar. "Cheney and Rumsfeld, they've been here," he says, gesturing to the bar. "And Ollie. We all worked here together years ago. She can even tell you what they drank." His eyes shift toward his wife, behind the bar. "When I used to run exercises we'd bring 1,000 people," he says. "Most of the things we did, they didn't let 'em off the post." He talks vaguely of one training exercise. "We had to do the psychology of being locked up," he says. "We started with submarines."

There have been curious visitors to Mount Weather from the start, he says, including the Russians. "The State Department, in their infinite lack of wisdom, allowed the Russians to have a R&R center on the river here, right below Mount Weather." The Curve, which sits off an entrance to the Appalachian Trail, attracts wayward visitors. "One hiker came in and said he was hiking all the facilities. Said you could get closer that way. He was trying to find out a little too much."

Local people, Wink says, like to help Mount Weather maintain its low profile. "They won't talk about it," he says. "As a matter of fact," he says, fixing his eyes on me, "you might meet a local cop if you ask too many questions about it. Many of the men around here served in the second world war," he continues. "Consequently they don't discuss those things."

I had encountered a similar line of thinking the night before from Derby, a long-time federal emergency coordinator and civil defence officer who is now retired and living in nearby Winchester. "All the employees of Mount Weather have always been told, rightly so, that no matter what someone asks you, just don't say if it's true or not true. Just ignore the question. You'll get that if you ask," says Derby, a chain-smoker with neatly Brylcreemed hair who drinks what he calls "martoonis" out of a tumbler. His office, in the upstairs of his split-level suburban home, is filled with various presidential commendations, as well as a photograph of what looks like a emergency conference room.

"I designed that," he says, peering through a dense curl of cigarette smoke, "but I can't tell you where it is".