"And you'll see a very long runway right...there." Our aircraft commander jabbed a finger at a small, cross-hatched circle on the U.S. Air Force navigation chart. "But, even if we lose all four engines," he said, "we will not land on it.”

“Why not, sir?” I asked.

“We’d be there a long time and have to answer a lot of questions,” the commander replied, then moved on to the next day’s mission preview. A four-stripe sergeant leaned toward me. “That’s Groom Lake,” he said under his breath. “That’s where the really secret [stuff] happens.”

As a 22-year-old airman, I thought our outfit’s nuclear-sampling missions were mighty secret stuff. Not until the 1970s, when the Air Force Technical Applications Center’s mission was declassified, could we tell our families that we had routinely flown a variety of aircraft—from C-54s to B-52s—through radioactive clouds from U.S. and foreign bomb tests, collecting nuclear debris and gaseous samples (see “Into The Mushroom Cloud,” Aug. 2009). We wore plain black baseball caps and olive-drab flightsuits with no unit patches, and carried only the most basic identification. Whatever was going on at Groom Lake evidently was even more classified than our mission. I was impressed—and very curious.

That preflight briefing, at McClellan Air Force Base in California in the summer of 1969, was my introduction to one of the most secret aerospace development and testing sites in the world. Over the next 40 years, the Groom Lake mystery surfaced time and again, first in my role as a flight test engineer and later as a reporter for the magazine Aviation Week & Space Technology. But, like every other journalist chasing the story, I have yet to set foot on the alkaline dry lakebed with ultra-long runways, despite multiple requests to visit.

The secluded desert base, nestled along the rim of Groom Lake, about 80 miles north-northwest of Las Vegas, has been a test site for intriguing, cutting-edge aircraft since 1955. It has also been a breeding ground for rumors and conspiracy theories. Until the 1990s, the U.S. government wouldn’t even acknowledge that the place existed.

This black hole of military and intelligence-community secrecy is a product of the cold war, when the United States desperately needed to spy on Soviet bomber and missile developments. In the 1950s, when satellites capable of watching airfields, missile silos, and test sites across the Soviet Union were years away, the Central Intelligence Agency awarded Lockheed’s advanced development division, by then known as the Skunk Works, a contract to build what became the high-altitude, long-endurance U-2 spyplane. Designer Clarence “Kelly” Johnson directed Lockheed test pilot Tony LeVier to find an out-of-the-way airfield where the U-2 could be flight-tested. It had to be remote enough to prevent Soviet agents from spotting the black, long-winged airplane, yet reasonably close to Lockheed’s southern California engineering and aircraft production plants.

LeVier scoured the southwestern United States in a light aircraft, ultimately settling on a dry lakebed deep within the nation’s Nuclear Test Site complex in Nevada, which the Air Force had opened to the Atomic Energy Commission in 1950 to conduct nuclear weapons tests. In the summer of 1955, facilities were quickly built along the south shore of Groom Lake, and a few months later, U-2 test flights began. Johnson thought the infrastructure would be temporary, but over the next 50 years, new buildings and longer runways were built to accommodate development of numerous secret aircraft, including Lockheed’s SR-71 Blackbird, its A-12 and YF-12 cousins, the F-117 Nighthawk stealth fighter, and one-of-a-kind technology demonstrators. An impressive fleet of foreign fighters and attack aircraft—primarily Russian—also were collected at Groom Lake, flown, and evaluated by a group of test pilots and engineers known as the Red Hats.

In Johnson’s time, Lockheed workers called Groom Lake “the Ranch” or “Watertown Strip.” Flight test professionals at Edwards Air Force Base in California, which maintains a unit at Groom Lake called Detachment 3, referred to it as “the Site.” U.S. and foreign pilots participating in large-scale Red Flag exercises flown from Nellis Air Force Base in Nevada know Groom as “the Box,” a no-fly zone that often complicates the massive air-combat training scenarios. To most of the public, it’s simply “Area 51,” a reference to a numbered section on 1950s Atomic Energy Commission maps that divided the Nevada Test Site into grids.

SINCE 2005, Groom has undergone a robust burst of construction. The base now comprises numerous large hangars, dozens of worker dormitories, unusually long, paved runways, acres of concrete ramp space, recreation facilities, and even a watering hole called Sam’s Place, I learned from base employees. Groom watchers have spotted an enormous new hangar boasting an estimated 65,000 square feet, sheltered by a roof that at its peak soars to about 100 feet. Naturally, the expansion has fueled considerable speculation among enthusiasts who try to track Groom’s goings-on, especially the black-program aircraft based there. One of my sources at the Pentagon said the base’s current fleet is dominated by unmanned aerial vehicles like the Air Force’s RQ-170 Sentinel. Dubbed the “Beast of Kandahar” by U.S. troops in Afghanistan who have seen it, the stealthy Sentinel—a tailless flying wing built by Groom Lake’s first tenant, the Skunk Works—is tailored for tactical intelligence, surveillance, and reconnaissance missions to support combat troops. The Air Force confirmed the Sentinel’s existence in December 2009, but details about the gray, unarmed UAV—which was first spotted at Kandahar Airfield in late 2007—remain classified.