Image copyright Getty Images Image caption A back gate to the top-secret US military installation in Lincoln County, Nevada known as Area 51

In Nevada's remote southern county of Lincoln lie two unassuming towns, their combined population, 173. Surrounded by arid landscapes and dusty roads as far as the eye can see, the towns of Rachel and Hiko appear unremarkable.

What makes them special, however, is their proximity to a top-secret US Air Force base, commonly known as Area 51. The mysterious military test area, long associated with UFO conspiracy theories, has cemented their place in alien folklore.

In these isolated towns, from 19 September, tens of thousands of people are expected to gather for two festivals, Alienstock and Storm Area 51 Basecamp.

Unlike most large-scale festivals, these events were not years in the making. Instead, they stemmed from an internet joke posted to Facebook just four months ago.

In a whirlwind few months, the person behind that joke helped organise one of those festivals, which he now fears could be a "possible humanitarian disaster".

In June, 20-year-old Matty Roberts, a student from Bakersfield, California, posted a tongue-in-cheek Facebook event.

The name of the Facebook event was: "Storm Area 51, They Can't Stop All of Us". The plan, as the name implied, was to charge at the base in large enough numbers to bypass security.

Once inside the facility, the supposed secrets lurking within - the alien technology and clandestine government research - could finally be disclosed to the public. "Let's see them aliens", the event's description declared, albeit flippantly.

Within days of its launch, the event became a viral sensation, making headlines across the world.

"I posted the Area 51 Facebook event at about 2am on June 27," Mr Roberts told the BBC. "It was totally a joke from the get-go. There was just a tonne of attention out of nowhere and it was awesome."

Image copyright Facebook Image caption Matty Roberts posted for Storm Area 51 Facebook event in June for a laugh

As it stands, more than 3.5 million people have expressed interest in attending the event on 20 September. Mr Roberts said there is a "good handful taking it seriously".

One of them is 33-year-old real estate investor Art Frasik, from Ohio. He told the BBC that he and others were determined to enter the facility to "expose and embrace the discovery of the aliens".

"I'm going into Area 51 because our American tax dollars funds this facility and after 70 years of hiding alien technology from the world, it's our right to see it," he wrote in a Facebook message.

Two men, a YouTuber and his friend, both from the Netherlands, have already been arrested three miles inside a restricted zone near the Area 51 base. Charged with trespassing on Tuesday, Ties Granzier, 20, and Govert Sweep, 21, told police they "wanted to look at the facility".

Image copyright Matty Roberts Image caption Matty Roberts said he started the Storm Area 51 Facebook event as a joke

Others have made similar threats to enter the base, yet alongside an abundance of memes, their seriousness is difficult to determine.

The US Air Force, on the other hand, is taking these threats seriously. Laura McAndrews, a spokeswoman for the Air Force, told the BBC that "any attempt to illegally access military installations or military training areas is dangerous".

Mr Roberts has echoed that warning, saying he doesn't want anyone to get hurt. After the Facebook event went viral, FBI agents knocked on his door to question his intentions. Mr Roberts assured the agents he "wasn't building pipe bombs or something insane".

But what began as his "funny idea for a meme page" has escalated beyond his control. For the US Air Force and the counties of Lincoln and Nye, Area 51 trespassers could be the least of their worries.

Image copyright Getty Images Image caption Matty Roberts, 20, is a college student from Bakersfield, California

In particular, the two festivals are posing major challenges for local law enforcement and infrastructure. Alienstock, hosted by the Little A'Le'Inn hotel in Rachel, and Storm Area 51 Basecamp, held at the Alien Research Center in Hiko, have both been granted permits by Lincoln County.

In preparation, the county has pre-signed an emergency declaration. Nobody knows with any confidence how many people will attend, but figures ranging between 5,000 and 50,000 have been bandied around.

Visiting Area 51 has been a pilgrimage for alien conspiracy theorists for decades. Located next to Groom Lake, the base was established in the 1950s as a test facility for the US spy plane, the Lockheed U-2 reconnaissance aircraft.

But according to Glenn Campbell, an Area 51 expert, the facility didn't come to be publicly associated with aliens and UFOs until the 1980s.

This was when physicist Bob Lazar came forward on a Las Vegas TV station. In an interview, he claimed he was hired to reverse engineer an alien spacecraft at a separate facility close to Area 51.

"With his announcement, thousands of conspiracies theories were born," Annie Jacobsen, who wrote a bestselling book on Area 51, told the BBC.

Because the base is a classified military facility, no-one knows what is currently going on there. Its purpose has always been to "advance military science and technology faster and further than any other foreign power's in the world", Ms Jacobsen said. Its veil of secrecy leads others to different conclusions.

"The base remains a black box that no-one can get into, so it has become a sort of Rorschach test for whatever you want to believe," Mr Campbell told the BBC.

Image copyright Reuters Image caption Area 51 is situated about 30 miles (48km) south of Rachel, in Lincoln County, Nevada

As Mr Roberts' Facebook event demonstrated, Area 51 remains a magnet for alien conspiracy claims.

Lincoln County sheriff Kerry Lee told the BBC that, even if 1% of the Facebook guests showed up, it "would be more overwhelming than we can handle".

Mr Lee said an additional 150 officers and 300 paramedics are being brought in from across Nevada. Usually, only 26 officers are at his disposal for the entire 10,000 sq mile (27,000 sq km) county.

Image copyright Getty Images Image caption An Extraterrestrial Highway sign covered with stickers is seen along state route 375

Those who do trespass on the Area 51 site will be arrested and could face a fine of at least $1,000 (£800), Mr Lee said. "My advice would be, if you're planning to come and see Area 51, you see it from outside the boundary," he warned.

Resources are being diverted from multiple local, state and federal agencies, including the FBI. The FBI, according to Mr Lee, is gathering intelligence on "the numbers that may be coming to these events".

If top-end estimates prove accurate, Lincoln County's population (around 5,000) would increase 10-fold overnight. In Rachel and Hiko, Mr Lee said, the amenities would be insufficient. Aside from a few shops and a hotel, their medical, restroom, gas and internet services are non-existent or patchy at best.

Image copyright Getty Images Image caption All rooms at the Little A'Le'Inn in Rachel, where Alienstock is taking place, are fully booked

A statement on Rachel's website predicted that Alienstock would be "Fyre Festival 2.0". Fyre Festival, held in 2017, was billed as a glamorous party in the Bahamas, but ultimately promised more than it could deliver. Some believe that Alienstock could suffer a similar fate.

Earlier this week, Mr Roberts officially severed ties with Alienstock, citing fears of a "possible humanitarian disaster". "Due to the lack of infrastructure, poor planning, risk management and blatant disregard for the safety of the expected 10,000+ Alienstock attendees, we decided to pull the plug on the festival," a statement on Alienstock's website read.

Former Alienstock organiser Frank DiMaggio also distanced himself from the festival. He told KTNV Las Vegas the event "needs to go away before it becomes the biggest disaster that southern Nevada has ever seen".

Connie West, owner of the Little A'Le'Inn, said Alienstock will go still ahead. She told KTNV she has paid for security, toilets and paramedics and, on Monday last week, announced 20 bands and two comedians on the line-up.

Image copyright Getty Images Image caption Connie West, owner of the Little A'Le'Inn, said Alienstock is still going ahead "no matter what"

When asked if she had any regrets, Ms West broke down in tears on camera. "No matter what it's going to happen, there's nothing I can do to stop it," she told a KTNV reporter.

Organisers of the second event, a two-day gathering of "believers", UFO experts and musicians at the Alien Research Center in Hiko, seem more sanguine.

Keith Wright, an organiser for Storm Area 51 Basecamp, told the BBC they have capacity for around 5,000 people.

Day passes for the event cost $51 (£41) per person, with parking, two bottles of water, a $10 food truck voucher included in the price. On-site accommodation is limited to tents (costing $50 per day) or parking for RVs and trailers (costing $150 per day).

Image copyright Alien Research Center Image caption Basecamp Area 51 is being held at the Alien Research Center in Hiko

Mr Wright, 49, said there's a "fair amount" of public space for tents, while water, electrical and sanitary needs are covered. But if the numbers exceed the tens of thousands "there's no way they can be taken care of by the existing infrastructure", he said.

Mr Wright, like Alienstock's organisers, knows the stakes of miscalculation are high. The consequences could be calamitous, as they famously were for Fyre Festival.

As the clock ticks, a sense of anticipation is building, though Mr Roberts said he has no regrets, yet. Anyone who attends Alienstock, or attempts to storm Area 51, is "doing so by their own accord".

"I've done everything in my power to try and warn against that," he said.

Unlike the US government, Mr Roberts has opted for full disclosure. The truth - from his perspective - is out there.