Marble Hornets was a web series that ran from 2009 to 2014. The story focused on a person called Jay, and his attempts to solve the mystery surrounding what happened to his friend Alex, and the film he was making before production was suddenly abandoned. He recieved Alex's filming tapes, and uploaded them into "Entries" on YouTube. Along the way, he encounters the mysterious masked men, the "totheark" channel uploading cryptic replies to the videos and the terrifying entity known as "The Operator".
Marble Hornets YouTube channel
Clear Lakes 44
Clear Lakes 44 is a follow-up/spin-off series of Marble Hornets, and seemingly takes place in the same universe. It started August 2015. Not much is known about the series yet, although it seems to take place before Marble Hornets. It seems to be more ARG-oriented than Marble Hornets, with many mysteries and puzzles left for the viewers to solve.
Clear Lakes 44 YouTube channel (same channel as Marble Hornets)
marblehornets.com, the website which serves as the main focus of the ARG.