The Princeton Experiment was a United States government-funded attempt at recreating the ancient Egyptian rite of Gorr'Rylaehotep at the Atlantic Test Site. The experiment began in 1956 and was allowed due to the fact that the United States government was attempting to gain an edge on the Soviet Union.
The program director of the experiment was Hans Freuhauf, a former Nazi scientist whom the United States acquisitioned to work on their new space program due to his research in space portals. When given control of the research division, Freuhauf attempted the Princeton Experiment with the aid of the information contained on the ancient Egyptian tablet discovered by Heinrich Kaufmann II, which fell into Nazi hands around the time of World War II. Others working on the experiment included Lieutenant Colonel Everett Walker (a military supervisor who was skeptical of the experiment), as well as Professor Gallagher and his assistant, Marc McComber.
The goal of the experiment was to create something known as a Construct in an effort to open a Rift and summon a Gorr'Rylaehotep. The experiment made use of criminals put on death row, who were considered to be essential materials, implying the criminals may have been harvested for their life energy. The first trial ended in failure and a magnetic field surrounded the area as a result.
The second trial, attempted in 1957, proved successful, as a Rift to another world was opened. However, the Slender Man appeared and slaughtered all who were present except for Marc McComber, who would have to deal with the results of his exposure to the phenomenon for the rest of his life. Although he was forced to agree not to disclose any information on the experiment, McComber later revealed said information to The Order.
In the 1990s, the experiment was the subject of the unaired pilot of Unsolved Conspiracies, which featured an anonymous appearance by McComber. In the pilot, McComber claimed that there were two groups that would want him dead for revealing such information, referring to The Order and the United States government.