The Ark is a location/object/person that is referenced multiple times throughout several ARG's relating to Slenderman. The actual existence of The Ark is unsure, as to what it is, why it is there, and how it has come to be are uncertain. Why it is so important to Slenderman isn't certain either, nor is how to find it or what it is capable of.
Marble Hornets internet stalker ToTheArk
ToTheArk is the only person in the Marble Hornets ARG that appears to be involved with The Ark. He never directly states what it is, where it is, but the audio in several of the videos seems to imply he wants Jay to lead him to it. Most people assume it is either Alex Kralie or Tim, but the identity of ToTheArk is still unknown, nor has Jay found anything that would be classified as "The Ark".
A potential theory is that "The Ark" is some form of acronym for a name, however who it would be is entirely debatable.
It is possible that Alex is "The Ark," following the acroynm theory. Alex R. Kralie.
Another theory is that Tot means dead in german... Tot heark could be a name: a dead man who is called heark the slenderman also comes from germany in schwartzwald
There is a video not part of the official video list on YouTube that features Vincent, Evan, and Jeff in the basement of Evan's house, where it is totally dark. Everyone seems to be injured with medical tape on their wounds and heavily on edge. Jeff notes that they haven't seen anyone in what they assume to be days. Electricity seems to be not working, the sun hasn't been out, according to Vincent, in "Three Days" while Jeff feels it has been "an hour". They mention having been in the company of a doctor (probably Doctor Corenthal) but he has gone missing. They are then attacked by The Rake in the basement, and try to flee, but just as Jeff gets taken the video cuts out.
Between the two videos, EverymanHYBRID has given more information, but due to the nature of Marble Hornets staying away from involvement with other ARG's, it may be different there. EverymanHYBRID seems to show that The Ark is a totally different dimension where time passage is off, clocks don't work, electrical equipment that doesn't have batteries has failed, there are no people, and it can be assumed The Rake has full and complete access. Due to this video being a video randomly posted outside of the EverymanHYBRID videos it can be either considered from the future or non-canon, however how the footage could have gotten recovered from The Ark isn't explained. One theory that could also coincide with the video is that it isn't our EverymanHYBRID crew, but another crew from another universe.
Due to the very limited nature of The Ark, little can be factually stated beyond concepts and assumptions.