While 1999's Blair Witch Project was the movie that kickstarted the found footage horror craze of the 2000s, it's Paranormal Activity, released a decade later, that has proved to be the subgenre's biggest longterm success. While Blair Witch remains a horror classic, it proved to be a one-off in terms of popularity, and the initial sequel and the more recent reboot were both commercial disappointments. Paranormal Activity, however, spawned an extremely successful franchise, with six movies so far and a seventh due next year.

So why did Paranormal Activity have such an impact when so many post-Blair Witch found footage horrors were quickly forgotten? Much of it is down to its pure, brilliant simplicity. Director Oren Peli was not some cynical producer looking to cash in on a popular craze. He was a software developer with no prior filmmaking experience, who simply had a great idea that was relatively easy to execute. A couple experience strange, scary noises in their house, so set up video cameras to record overnight to find out what is causing them. The tension that builds as we watch this nighttime footage, waiting for something sudden to happen, is masterfully done. It's so simple and effective that it makes you wonder why it hadn't been done dozens of times before.

Inevitably, the Paranormal Activity sequels have varied in quality, and while they all have effective moments, none of them quite capture the scares of the first movie. They are all part of the Hollywood horror machine, something the original film absolutely wasn't (initially at least). And the story behind its journey to the screen is a fascinating one, with festival rejection, different endings, a delayed release, and ultimately, a huge commercial victory for the filmmakers. Paranormal Activity can be streamed right now on Netflix, and here's our guide to this terrifying modern classic.

1. Director Orin Peli was inspired by late night noises

Peli's inspiration to make the film came from hearing strange noises at night when he moved into his new house. While he didn't think the house was actually haunted, he told Chris Jones in 2010 it gave him the "idea of what if someone did think their house was haunted and wanted to prove or disprove it, so they set up a video camera."

2. Peli made the movie cheaply and quickly in his own home

The movie was actually shot in 2006, three years before it would eventually be released. It was filmed in just seven days, in Peli's own San Diego home, and cost a mere $15,000 to make. Peli paid for the movie himself and simply took a week off work to get it done. He didn't even tell his neighbors he was making a film in his house.

3. There wasn't a script

The naturalistic performances in the movie are down to the fact that there wasn't a traditional script and the actors were mostly improvising from a basic outline. Peli explained, "The treatment was like: a couple goes to sleep, such and such happens and then they investigate. As we were getting closer, I refined it more and as we were shooting it was really brought to life by the actors."

4. Peli reworked his house for the film

Ahead of filming, Peli made a series of changes that would help his house look more interesting in the film, such as switching the carpets for hardwood floors, painting the walls, and replacing the stairs. He subsequently stated that while he didn't include these in budget as they were planned improvements, "the hardwood floors alone cost more than the whole film."

5. Peli wasn't crazy about the title

The movie didn't have a title for a long time, but the filmmakers had to finally pick one when they started submitting it to film festivals in 2007. Peli admitted that he didn't even like "Paranormal Activity" that much, but at least "it didn't suck and it's dry and accurate."

6. It was rejected by most film festivals

Peli submitted the movie to a number of major film festivals, including the Toronto International Film Festival, but it failed to be accepted by any. Only one festival accepted it--the much smaller Screamfest in Los Angeles. But the reaction it got from audiences and critics there was enough to get the Peli noticed by major talent agency CAA, who subsequently sent out DVDs of the movie to interest a studio in buying it.

7. It was Jason Blum's first big horror hit

Paranormal Activity was the first big success for producer Jason Blum, whose Blumhouse Productions has gone on to become one of the biggest companies in horror, making such hits as The Purge, Get Out, and the most recent Halloween movie. Blum was working at Miramax when he saw the first cut of the movie on DVD, and was impressed enough to come on board as a producer, helping Peli re-edit the movie and eventually score a deal with DreamWorks, the studio co-founded by Steven Spielberg and owned by Paramount.

8. The studio wanted to remake it

We almost never even got to see this version of Paranormal Activity. After Paramount purchased it, the studio wanted Peli to entirely remake it with a higher budget. Peli and Blum agreed, but asked for a test screening, knowing how well the original version might play to an audience. They were right--the audience response quickly convinced the studio that there would be no need to make it a second time.

9. There are three different endings

There are in fact three endings to the movie. Peli's original version, which was screened at film festivals, ended with Katie being shot by police as she emerged from her bedroom holding a knife. When the movie was bought by Paramount, two new endings were shot. In the one that became the "official" ending, Micah is killed by being hurled across the room by some unseen force, and Katie turns into a snarling demon. In the third ending, which was never released as part of the movie but can be found as a DVD and Blu-ray extra, Katie kills Micah offscreen and then cuts her own throat in front of the camera.

10. It's one of the most profitable movies ever made

The huge success of Paranormal Activity makes it one of the most profitable films of all time, in terms of a percentage return on investments. The final budget was more than the initial amount that Peli paid to make it--Paramount paid $350,000 for the rights, and additional post-production and new shooting added an additional $215,000. Nevertheless, with a worldwide box office of $193 million, there's no denying what an incredible investment it was for everyone involved.