1. The May Day Mystery The May Day Mystery / Via maydaymystery.org The theory: In 1997, Bryan Hance, a student at the University of Arizona, began to investigate cryptic ads that run every year on 1 May in the student newspaper, The Daily Wildcat. After a bit of research, he found out the ads go back as far as 1981, potentially further. It's still going on today, and you can see the 2016 May Day ad here. The theory is that the ads are communication between a group of intellectuals, and Hance believes the ads contain meeting dates, past and future, for a secret society that is planning for an economic and political revolution. Hance tracked the ads for the last 10 years to a lawyer, Robert Hungerford, who claims to have no involvement with the creation of the ads, but says he is just asked to send them to publish.Any proof? Most ads feature incredibly obscure historical references, symbology, and mathematical calculations, so it's likely they aren’t just nonsensical ramblings but are the result of a deliberate, careful effort. Hance and a friend working with him were contacted in 1999 by a member of a group called The Orphanage, saying that the adverts are part of a larger cause, and the group have supposedly sent him clues and donations over the years. A letter to Hance from The Orphanage ends with the sentence, "The day you can see the door, you will be welcomed inside." 2. Phantom Time Hypothesis Thinkstock The theory: Roman Emperor Otto III misdated the Western calendar 297 years because he liked the idea of ruling in the year 1000 AD. Otto, the Pope, and others filled in this "phantom time" with some completely made-up history: the Early Middle Ages. If this is true, the current year is actually 1719.Any proof? Surprisingly, yep. There are literally thousands of recognised forgeries of documents from the early Middle Ages that claimed to be written hundreds of years before they were, and describe events in detail, centuries before they happened. A huge majority of these were made by the Church. There is also hardly any literature, art, records, or cultural artefacts from Western Europe in this period, nor any real progress in agriculture or technology – hence the term "the Dark Ages". Read a paper about it here. 3. The Codex Alimentarius Thinkstock The theory: Internationally recognised food standards and dietary guidelines, called Codex Alimentarius, are actually a method of "soft-kill eugenics" designed to kill a bunch of us off to reduce the planet's population down to a more manageable level.Any proof? Well, the first warning sign is the dodgy name: Codex Alimentarius sounds pretty sinister (though it's just Latin for "food code"). Believers claim the codex tries to hide the benefits of herbal medicines and will restrict access to vitamin and mineral tablets. They also say the codex has renamed over 300 known poisons as safe food additives. 4. Chemtrails Fabio Macor (CC BY-SA) / Via flic.kr The theory: Since about the mid-'90s, some people, including some more famous believers, believe the cloud-like vapour trails emitted from planes – contrails – are actually chemtrails. Chemtrails supposedly are vaporised chemicals being sprayed into the atmosphere that aim to make the population ill and control the weather. There's a sizeable campaign in the UK devoted to raising awareness of it.Any proof? The theory was kicked off after the United States Air Force wrote a strategy paper that outlined possible ways the military could theoretically modify weather. Photos of planes with barrels installed in the passenger space (said to be "dispersion systems") are available online, but their purpose is actually to simulate the weight of passengers on test flights. 5. Reptillian Elite Ayesha Mittal for BuzzFeed / Thinkstock The theory: Shape-shifting reptile aliens control Earth by taking on a human form and gaining political power to manipulate humanity. "They" are the royal family, European aristocracy, and various global leaders — including David Cameron, and probably Theresa May — and the rest of us are humble "sheeple". This theory is rooted in ideas from the 1888 book The Secret Doctrine, which is a study on the origin of the universe and humanity. It's not exactly a "fringe" theory either as some 12 millions Americans believe the US government is run by lizard people – or reptilians.Any proof? Pretty much everything – e.g., the NSA, fluoridated water – is "evidence". David Icke argues that they altered our DNA, making us easier to control, and points to humans having reptile genes in our brains as evidence of this. In the 1960s, a police officer claimed to be abducted by reptile-like aliens. There are also a lot of videos of people claiming to have footage of reptilians shape-shifting. 6. HAARP and Weather Control Michael Kleiman (Public Domain) / Via commons.wikimedia.org The theory: The High Frequency Active Auroral Research Program (HAARP) was actually a weather-control facility, surveillance centre, and mind-control hub. Its official purpose was to analyse the ionosphere. HAARP had a main facility in rural Alaska, with hundreds of large antennae rising from the ground, forming what looked like a "bionic forest", which seems to have fuelled a lot of the conspiracies.Any proof? Dr Rosalie Bertell, an American scientist specialising in the ionosphere, warned about the usage of HAARP as a military weapon, writing that it could "deliver a very large amount of energy, comparable to a nuclear bomb". There have also been a bunch of claims that it's triggered various recent natural disasters, though none with any proof. 7. Flat Earth Theory Orlando Ferguson / Wikimedia Commons (Public Domain) / Via en.wikipedia.org The theory: The Earth is not spherical, it's just a straight-up flat disc. We all believed this theory until Pythagoras, (and later, Aristotle) introduced the Spherical Earth model. We had no way to prove it, because we didn't have the technology to fly up high enough to give us a true perspective of Earth. In the midst of the nuclear, arms, and space race in the 1950s, the US and Russia realised they were completely wrong about the Earth's shape. Except by then people had believed the Spherical Earth theory for centuries, and they were in way too deep. Global powers have since kept up the lie through faked space exploration and doctored photos of "the globe." The US Government uses the funding NASA gets for space missions to instead maintain huge power and wealth.Any proof? There are so, so many videos "disproving" that the Earth is spherical. Flat Earth Society claim to have testimonies from several pilots saying that no curvature can be seen at Commercial Airline heights. They also say that the horizon always rises to meet eye level, which is impossible on a spherical earth. They question why there are so few direct flights in the Southern Hemisphere, and why flight paths in that region are so complicated. "Flatists" argue the Antarctic Treaty of 1961, which made the Antarctic uninhabitable and unownable (and still applies today) was to ensure no-one goes to the "edge" of Earth. 8. The Overthrow of Sukarno Keystone Features / Getty Images The theory: The British Foreign Office was (partially) responsible for the bloody overthrow of Indonesia's first president, Sukarno. The office has always denied that it had anything to do with it.Any proof? A British ex–Foreign Office agent worked with MI6 and GCHQ in Indonesia in the early 1960s. He says he was given a budget of £100,000 and told to do pretty much anything to get rid of Sukarno. The CIA created a fake pornographic film with a Sukarno lookalike in an apparent attempt to discredit him. Yes, really. 9. Denver Airport Is a Front Mark Frauenfelder (CC BY 2.0) / Via flic.kr The theory: The Denver Airport, which opened in 1995, is a front for an underground military bunker. Any proof? Well, first off, there's the terrifying murals depicting mass genocide that are throughout the airport. It also was around $2 billion over budget, and it's not clear what that money was spent on. It's nearly twice as large as the next biggest US airport, and its funder – the New World Airport Commission – doesn't seem to exist. It also has a dedication stone with a Masonic symbol on, which is really not helping with the whole conspiracy theory thing. 10. The Philadelphia Experiment National Archives and Records Administration (Public Domain) / Via commons.wikimedia.org The theory: The US military allegedly carried out experiments in October 1943 where the US Navy USS Eldridge, stored in Philadelphia Naval Shipyard, was made invisible. Not only that, but it appeared in the seas around Norfolk, Virginia, some 200 miles away. This was part of an experiment to make ships invisible to enemy devices. Crew members reported nausea, some sailors became embedded in the ship structure, and others went mad.Any proof? Carlos Allende, a sailor, claimed to have seen the ship appear (and disappear again) in Norfolk but didn't have any proof. The Navy did conduct experiments in the 1940s to make ships "invisible" but with a completely different set of desired results. 11. RAND Conspiracy Theory Alltime10s / Youtube / Via youtube.com The theory: The think tank RAND is working on a new world order and basically controls all of us. It has steadily used its influence to replace rights with choices since the 1950s and aims to turn the world into a completely rational one, where love and selflessness no longer exist. It uses manipulative techniques to control dissent and create a "consensus" that works in its favour. It's part of the wider Illuminati conspiracy, where RAND is a tool to implement propaganda and social control, to help the Illuminati carry out its plans of an authoritarian world government.Any proof? RAND has been a major player and influencer in government decisions, policy, and development for decades. It's funded by a load of national security agencies and the largest corporations in the world. Plus it's really hard to nail down exactly what its views are. This all combines to create a perfect storm for conspiracy theories.