Signs at the entrance to Area 51, Groom Lake, Nevada

Most people understand that you can’t simply walk into a U.S. military base as if you own the place. There are sure to be checkpoints, security screens, and queries about your reasons for being there in the first place. But, so long as you have business inside and don’t seem suspicious, you’ll probably get in. That is, unless you try to get into some of the most exclusive military facilities in the country, which won’t let you pass a mere checkpoint or two onto what can be some seriously sensitive ground.

In fact, when it comes to some military installations, simply looking too long at some of them may soon put you face to face with security contractors who will have a lot of questions for you. These places are pretty well buttoned up, to say the least. And, with that level of secrecy and suspicion naturally comes plenty of curiosity and interest on the public’s part. Who doesn’t love a good mystery, after all? And, when it comes to U.S. military facilities, there are still some pretty mysterious ones out there, even today.

Fort Detrick is home to a top secret biological weapons program

Million-liter test sphere, Fort Detrick, Maryland, USA

From the outside, Fort Detrick may look like a pretty standard military installation in Frederick, Maryland. Of course, you’re probably only getting a glimpse of the base, which covers a sprawling campus that, both historically and today, hosts one of the United States military’s most terrifying and, by necessity, secret initiatives: biological weapons.

It all started, more or less, in the 1950. According to Politico, that’s when the U.S. Army determined that Fort Detrick, a mere 50 miles or so from Washington, D.C., would be the place where the military should begin developing and testing biological weapons. It was, at the time, kept top secret as scientists created bioweapons using agents and pathogens including anthrax, botulism, and tularemia. The program was retired in 1969 and replaced by a bioweapons defense program, still active today (via PBS).

Politico reports that it was also the central base for the CIA program known as MKUltra, a project in which sometimes unwitting test subjects were given LSD and other mind-altering drugs in an attempt to develop mind-control tactics. The program, which often sounds like science fiction more than fact, remains mysterious today in large part because many of the program documents were destroyed in the 1970s. Now, as Popular Mechanics reports, Fort Detrick is home to a state-of-the-art facility meant to contain all of that potentially deadly research material.

Cheyenne Mountain Complex is famously mystifying

NORAD blast doors, Cheyenne Mountain, Colorado

If you were to bring up mysterious U.S. military facilities, chances are good that someone will bring up the Cheyenne Mountain Complex. The origins and operations of this base near Colorado Springs, Colorado are so notoriously peculiar that it’s since become a standard for hush-hush military installations.

Given the history of the place, it’s understandable why so much of the facility’s operations have been kept quiet over the years. According to Wired, the true heyday of Cheyenne Mountain came shortly after its completion in the 1960s, at the height of the Cold War. The complex is buried deep into the side of the namesake mountain and, with its complex of tunnels and life support systems, provided a pretty attractive bunker in the event of a nearby nuclear attack. The shielding provided by the granite does much of the work, along with earthquake-resistant architecture inside and plenty of shielding for all of the electronics.

Though reporters are occasionally invited inside for a limited tour of the facilities, much of what goes on inside Cheyenne Mountain Complex is still shrouded in secrecy. It needs to be, given that the base once housed operations for the North American Aerospace Defense Command (NORAD) and other monitoring and defense networks. Now, according to NORAD, Cheyenne Mountain is a backup command and training center, though many wonder if such an extensive and heavily shielded facility is really being allowed to sit more or less dormant in a tumultuous world.

Groom Lake and Area 51 are the focus of many UFO rumors

Signs at Area 51 gate

A military base so notorious you can’t wade through a UFO conference without hearing it mentioned a few hundred times, the installation known as Area 51 remains locked down. In part, that’s because it’s located in the remote Nevada desert, per CBS, but officials have also gone to great lengths to keep mum about it. It’s been so utterly secret that the U.S. government didn’t even acknowledge the base’s existence until 2013.

Many speculate that this is because Area 51, known officially as Groom Lake, is all about espionage. Consider that, if you were a government official who managed a program meant to monitor Soviet activity during the ultra-tense days of the Cold War, you probably wouldn’t want to publicize your work, either.

That secrecy can start to encourage some pretty wild conspiracy theories over the years. Some of the more mundane ones speculate that Area 51 is a top secret testing ground for espionage equipment and high tech aircraft. Meanwhile, as Vox notes, the more out-there theories maintain that it’s not just spy planes at Groom Lake, but instead extraterrestrial craft, perhaps even complete with alien pilots. Still, with security contractors patrolling the perimeter and access to the site highly restricted, no one on the outside knows for sure, making Area 51 still one of the most mysterious military facilities in the entire nation.

NSA Utah Data Center is a mysterious espionage base

NSA Utah Data Center, February 2019

As far as mysterious military installations go, it makes sense that any place dealing with high-tech intelligence and surveillance would be pretty locked down. Certainly, after high profile leaks from former contractors and military employees like Edward Snowden and Chelsea Manning, you can be pretty certain that security measures around these places are at a high level. And that’s precisely why some intelligence facilities are so mysterious and compelling to those of us on the outside.

Take the Utah Data Center operated by the National Security Administration. Many have wondered why it would be located in Utah, The Atlantic reports, though the state certainly has plenty of remote areas and a distinct lack of natural disasters. Local communities have also given the NSA a pretty sweet deal on water rights and other infrastructure needs.

And what are they doing at the NSA Data Center? All anyone seems to feel confident about is that the data center likely stores massive amounts of data collected via any number of undisclosed methods. Quite a lot of that data, many assume, is harvested directly from American citizens and their communications, as NPR reports. While administrators have plainly stated that they don’t hold such data and instead focus on foreign targets, the lack of detail has kept many still thoroughly skeptical.

Mount Weather has been a mysterious military site for many years

Looking west from the site of the crash of TWA flight 514 at Mount Weather, Virginia, in 2017

The nature of a top secret military bunker, especially one that might be used to house big-name government officials in the wake of a national or international disaster, is that it must be kept, well, secret. It may seem like a fun mystery to people on the outside, but the very nature of such an installation means that people in the know really, really need to keep their mouths shut.

That’s a large part of why people are so interested in Mount Weather, the highly secretive military installation about an hour from the heart of Washington, D.C. In part, that’s because, as The Guardian reports, Mount Weather is a bit of an open secret. It may have been a bit more concealed in previous years, but the events of September 11, 2001 led to a visible flood of government officials and their vehicles into the area, drawing some pretty serious attention.

Reports from over the years hint at a sprawling, well-supplied complex, complete with an independent electrical system and, of course, a helipad. Others strongly hint that Mount Weather is just one of a series of similar sites. Yet, that’s just about all civilians seem to really know about the place. The perimeter is heavily controlled and anyone who attempts to breach the fence would surely meet some truly serious resistance.

Diego Garcia allows the U.S. military to operate secretive interrogations

Satellite image of Diego Garcia atoll

Though many U.S. military bases and facilities are, naturally enough, within the mainland United States, overseas territories and small landholdings make up some of the most mysterious entries in the shadowy military base lineup. And top amongst those is a base located on a small atoll many, many miles away from the U.S.

That atoll, according to CNN, is actually part of the Chagos Islands in the Indian Ocean. The base in question, Diego Garcia, exists in part because of a unique agreement between the United States and the United Kingdom. In 1965, the U.S. agreed to lease the atoll from the British to create a base, but the arrangement was kept strictly secret. And while other nearby nations like Mauritius gained independence from the British Empire, the agreement basically meant that the Chagos Islands are still under the thumb of colonial British powers. And the Chagossians who inhabited the islands? They were kicked off to make way for the Americans and their secretive facility.

Though the agreement is now out in the open, the exact operations of Diego Garcia are still pretty murky. Part of that’s due to the pure physical distance of the place from anywhere else. But, per CNN, the British government has admitted that the CIA has flown in detainees, making some wonder if the remote nature of the base has allowed it to become an alarmingly secretive version of Guantanamo Bay.

The HAARP Research Station is the subject of many conspiracy theories

HAARP antenna array

When it comes to conspiracy theories centering on the U.S. military, a program known as HAARP is sure to get quite a few hackles raised amongst believers. As NBC reports, HAARP is short for High Frequency Active Auroral Research Program and is ostensibly a government-run research effort to study our planet’s upper atmosphere. But some have alleged that HAARP is actually there to create or manage weather-based weapons systems that can cause massive storms and deadly earthquakes. That said, no one on the conspiracy side has come up with a good explanation for how a weather station located in Alaska could be responsible for something like earthquakes in Haiti.

Though the military claimed it was only undertaking the project because it was largely interested in how radio signals traveled through the atmosphere, its remote nature and military security made many suspicious. Even now that HAARP has been transferred from the U.S. Air Force over to University of Alaska Fairbanks, some still cast a wary eye at this remote and quasi-mysterious facility.

Few are sure of what’s going on at Raven Rock Mountain Complex

Raven Rock Mountain Complex logo.

If you thought a name like "Cheyenne Mountain Complex" sounded pretty cool, then you’ll probably be interested in the Raven Rock Mountain Complex on name aesthetics alone. In another context, it sounds like it could be the title of a pretty groovy prog rock album. But the reality of Raven Rock is far more locked down and, some might say, pretty sinister.

Not unlike Cheyenne Mountain or Mount Weather, reports strongly indicate that much of Raven Rock Mountain Complex is underground. According to Atlas Obscura, the site also goes by the monikers "Site R," the "Alternate Join Communication Center," and, more colloquially, the "Underground Pentagon." As you may well guess, Raven Rock looks to be yet another extensive government bunker that was created in the high-tension years of the Cold War, when U.S. officials fretted over the threat of nuclear annihilation, courtesy of the Soviet Union. Raven Rock, located in Fairfield, Pennsylvania, appears to be the retreat site for Pentagon personnel.

Though Atlas Obscura reports that the site’s construction was covered by local press during the 1950s, Wired notes that Raven Rock today is still covered by a veil of secrecy. Sure, military representatives will send over unclassified information to reporters, but what about all the classified stuff? No one but a small group of workers there knows what exactly is in the bunker, though Wired argues that perhaps it’s time we discarded the idea of bunkers, anyway — a modern warhead with the coordinates to a known site like Raven Rock could easily destroy the complex.

The CIA keeps Harvey Point mysterious on purpose

U.S. Navy Sea-Air-Land (SEAL) team members wearing chemical-biological masks train in the Southern California desert.

People don’t go to Harvey Point without a purpose. And, given that the CIA has a pretty heavy hand in the operations of this facility, you can be sure that quite a few people who work at Harvey Point are tasked with some pretty serious business.

Technically called the Harvey Point Defense Testing Activity facility, the base is ostensibly operated by the Department of Defense in eastern North Carolina. But, according to The New York Times, most people assume that it’s really run by the Central Intelligence Agency, which tends to keep many of its activities tightly locked down and so wouldn’t necessarily publicize a high-security training base. Other participating agencies are believed to be the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the Federal Bureau of Investigation (FBI).

And who’s getting trained there? It appears to be counterterrorism forces, including, at least according to HuffPost, the Navy SEALS who carried out the strike that killed Osama Bin Laden in 2011. The New York Times also reports that the airspace above the facility is highly restricted, though images indicate that there are at least two landing fields there — leading some to believe that aircraft testing or training may be happening there as well.

Wright-Patterson Air Force Base is host to eerie aviation mysteries

Wright-Patterson Air Force Base - USAF Museum Buildings

If you ever find yourself in a gathering of UFO enthusiasts and researchers, you can certainly bring up classic topics like Area 51. But if you want to really make an impression, then you should start asking about Wright-Patterson Air Force Base.

Sitting in the rather unassuming city of Dayton, Ohio, Wright-Patterson AFB has long been the subject of mystery and conspiracy theories. And some of those theories are linked to verifiable facts. As the Dayton Daily News reports, Wright-Patterson was the host site for Project Blue Book, the government investigation intended to investigate UFOs and which officially ended in 1969. There’s even a series of murals on base painted by German POWs that include "green goblin-like creatures" that some have taken to mean that the Germans somehow witnessed "little green men" from outer space.

Central to many theories is the maybe real, maybe mythical "Hangar 18" somewhere on base. According to History, some of the bolder conspiracy theories hold that Hangar 18 houses no less than debris from extraterrestrial craft and maybe even the nonhuman pilots themselves, which may be linked to the 1947 crash in Roswell, New Mexico. However, the Air Force has repeatedly denied these claims, saying that there has never been a Hangar 18 (though there is admittedly a "Building 18" on base). Plus, no one’s come forth with definitive proof for these claims, meaning that it will all have to remain speculation for now.

Dugway Proving Ground is a test site for biological and chemical weapons

Dugway Proving Ground panorama

While some people might think that they could potentially get onsite of some of these facilities and at least survive their experience (however misguided and packed with prison sentences and fines that may be), some military facilities are off limits for very, very good reasons. It’s one thing to lark about just outside the gates of Area 51, for instance. It’s something else entirely to visit another base and get a bad case of anthrax. Or, apparently, aliens.

Located in a remote section of Utah, Dugway is run by the U.S. military with the intent to test chemical and biological weapons. As such, access to the estimated 800,000 acre site is strictly controlled, as The Guardian reports. Besides the extensive laboratories on site, there are also training grounds, including a series of linked shipping containers that have been modified to appear like cramped cave environments.

Though little is known about the exact nature of testing at Dugway, there have been multiple incidents that have marked the facility’s past. According to Smithsonian Magazine, around 6,000 sheep in the area were killed in March 1968. They were done in by a jet spraying nerve gas at too high of an altitude. The gas dispersed beyond the grounds and coated grass that was later consumed by the doomed sheep. More recently, a lab at Dugway inadvertently shipped live samples of anthrax across the country, raising considerable alarm (via The Washington Post).