Ivan T. Sanderson founded the Ivan T. Sanderson Foundation in August of 1965 on his New Jersey property. In 1967 the Society for the Investigation of the Unexplained (SITU) was founded by Ivan T. Sanderson on the suggestion of Loren Coleman, as a successor to the Ivan T. Sanderson Foundation. 

SITU was a non-profit organization that investigated claims of strange phenomena ignored by mainstream science and was dedicated to compiling and studying reports and other material related to unexplained events, sightings, and experiences, particularly those also ignored by conventional scientists. SITU's mission extended beyond cryptozoology to encompass a wide range of anomalous phenomena, from UFO sightings to alleged psychic abilities to the Bermuda Triangle among others. SITU was a hub for researchers and enthusiasts alike, providing a platform for the exchange of ideas and evidence related to the unexplained. The organization's interdisciplinary approach reflected Sanderson's belief that the mysteries of our world required a holistic investigative method, one that drew from various fields of study. The organization was dedicated to "the acquisition, investigation and dissemination of information on reports of all tangible items in the fields of chemistry, astronomy, geology, biology and anthropology, that are not readily explained," with a particular focus on cryptozoology. For a generation it was the leading organization pursuing research on anomalous phenomena of the kind usually associated with Charles Fort. It encouraged fieldwork and on-the-spot investigation by offering advice, helping to raise funds, and arranging contacts for members who planned field trips and expeditions. Fieldwork and research were reviewed by a panel of 20 scientists.

The society disseminated information on findings through a quarterly journal named Pursuit and through papers and reports. Investigations by society members included such areas as claims of ancient Egyptian television, ringing rocks, entombed toads, and poltergeist manifestations. The society maintained information files of original material, a map collection, and a specialized library. The material acquired by the Society was kept in a concrete building on Sanderson's own property. In his later years, it is alleged that a number of people visited him to "look" at the files, only to walk away with them: it was rumoured that "people in station wagons were backing up to the concrete bunker and loading books and files into their vehicles", and by the time of Sanderson's death, the SITU collection was heavily depleted.

The Society for the Investigation of the Unexplained was disbanded in the 1980s.