Conspiracy theories usually reside in some pretty dark corners of the internet, but every now and then one will become part of the mainstream.
And conspiracy theories have been around for thousands of years — look no further than Jesus Christ himself for speculation about his relationship with Mary Magdalene. Also, ask anyone with a passing interest in the assassination of John F. Kennedy about the grassy knoll, and you'll need to prepare for a torrent of information and conjecture.
Keep scrolling to learn more about these historical figures that have been followed, some for centuries, by wild conspiracy theories.
1. The most prevalent conspiracy theory about Abraham Lincoln is about his assassination — namely, that John Wilkes Booth didn't act alone.
The official record states that Abraham Lincoln was shot at Ford's Theatre on April 14, 1865, in Washington, DC, by John Wilkes Booth. Not everyone's convinced, though.
According to the Ford's Theatre website, there have been plenty of alleged co-conspirators in the plot to assassinate Lincoln, including Confederate President Jefferson Davis, Confederate Secretary of State Judah P. Benjamin, the Pope, and Secretary of War Edwin Stanton.
2. Amelia Earhart disappeared and was presumed dead after her plane went missing, but some aren't so sure that's how it went down.
Earhart, a prolific pilot, vanished in 1937 during an attempted flight around the world. Earhart and her navigator departed from New Guinea on July 2 and were never heard from again. Two years later, they were officially declared dead.
From then on there have been multiple theories surrounding what happened to her. For example, one theory posits that she was captured by the Japanese, because a photo surfaced in the National Archives of a woman's back that resembles Earhart. Japan denies this.
Another theory suggests that Earhart crashed, was captured by the Japanese, rescued by the US, and then moved to New Jersey to take up another identity, as per the book "Amelia Earhart Lives."
Unfortunately, the most likely theory is that navigator Fred Noonan and Earhart's plane crashed and the two were tragically killed.
3. John F. Kennedy's assassination is another event that's rife with conspiracy theories.
In American history, there may have been nothing more contentious than the death of JFK in Dallas, Texas, in 1963. You might have even heard buzzwords like grassy knoll, umbrella man, and the Zapruder film. Here's what they actually mean.
First, the Zapruder film: A bystander at the fateful motorcade happened to be recording footage of the president driving by. Conspiracy theorists believe that the film shows that multiple shots were fired, and that at least one was shot from a different angle than the other three, leading us to the grassy knoll.
The grassy knoll refers to a nearby grassy hill that another shooter, besides Lee Harvey Oswald, is theorized to have been lurking at, and that's where another mysterious shot supposedly came from.
Another theory, the umbrella man, refers to a man holding a suspiciously large black umbrella on a notably sunny day. As The Washington Post reports, some believed that this man was working with the perpetrator[s], and had somehow converted his umbrella into a dart gun meant to paralyze the president.
4. Many people believe that William Shakespeare didn't actually write his own plays and sonnets, and was instead just a figurehead.
Could it be true that Shakespeare, the most influential playwright in history, didn't actually write anything? Potentially ... at least 70 other potential candidates have been put forth over the centuries, but a few have become front-runners.
Sir Francis Bacon was the first alternate Shakespeare to be named by author Delia Bacon (no relation). Bacon, unlike Shakespeare, was well-educated, well-traveled, and an accomplished philosopher. According to Delia, the scholar would've sullied his reputation if he had openly written plays like Shakespeare's.
Two other popular theories are that Edward De Vere, the 17th Earl of Oxford, is the actual Bard, or that Shakespeare was really Christopher Marlowe. Proponents of this theory, called Marlovians, believe that Marlowe faked his own death in a bar fight, and then began writing in earnest.
5. At least one book has been written that claims "Alice in Wonderland" author Lewis Carroll might have moonlit as serial killer Jack the Ripper.
This conspiracy theory began with a book called "Jack the Ripper: Light-Hearted Friend," written by Richard Wallace, a "clinical social worker and part-time Carroll scholar," according to Mental Floss.
Jack the Ripper was a London-based serial killer who is said to have murdered at least five women in 1888. The culprit was never identified, leaving the case wide open for conspiracy theorists.
Wallace's theory rests on the idea that Carroll had a mental breakdown while he was away at boarding school, and that he was never able to recover from the trauma. Most of the "evidence" comes from re-arranging the nonsensical passages of "Alice's Adventures in Wonderland," into more sinister sentences.
6. A persistent theory about Jesus is that he was actually married to Mary Magdalene. This was popularized by Dan Brown's novel "The Da Vinci Code."
One theory about the crucial Christian figure that has had a resurgence as of late is that Jesus was married to — and had children with — Mary Magdalene.
Magdalene was a companion of Jesus', according to biblical writings, but there's nothing to suggest that their bond was romantic in any way — or at least, there wasn't until the Gnostic Gospels were found in Nag Hammadi, Egypt, in the 1940s.
These gospels appeared to confirm that Jesus and Magdalene were more than friends, and mention him kissing her frequently. However, many people disregard the Gnostic Gospels and don't consider them a reliable source, and the theory died out for a few decades.
It came back to life when "The Da Vinci Code" was published in 2003. The entire plot hinges on the idea that Jesus and Magdalene were married and secretly had children, and that their lineage continued on to this day.
7. According to some, Nikola Tesla invented what's been called a "death ray," and the US government has the plans.
When Tesla died in January 1943, the US government took a bunch of papers from his hotel room, and some claim that these included plans for a "particle-beam weapon," aka a death ray.
For decades after, nobody knew what the government did with all these documents, making it easy for people to believe that the authorities were allegedly hiding schematics for a death ray.
The FBI eventually released some of these documents, but many are still missing — and it's anybody's guess what's inside.
This article originally appeared on Insider. Follow @thisisinsider on Twitter.