Marble Hornets (abreviated to MH) is a hit-horror video series, started on June 20th of 2009 and ending on June 20th of 2014.
The main storyline has a run time of almost 9 hours, consisting of 3 seasons, every episode ranging between 5 to 15 minutes, titled "Entries" and ranging from 1 to 87 (All Entries can be found on the channel Marble Hornets). Aside from the main youtube channel, there's additional content in the totheark Youtube Channel and in the in-universe twitter Marble Hornets.
The first ever post about MH was created on June 18 of 2009, in the Something Awful Forums. It's a in-character post, where the user ce gars talks about a failed film-project done by his friend in college, Alex Kralie, that he got reminded of by the original slenderman post (posted on June 10 of the same year in the same forum).
Two days later, the first ever video is uploaded to the youtube channel on June 20, following the investigation of ce gars (AKA: Jay Merrick) into the strange circumstances that led to the failed film-project Marble Hornets and the mysterious appareances of a tall strange figure in the background of the film.