It is a small cellar containing a single mattress, a safe and various pre-War foods strewn across the floor and shelves, built by the owner, Jahani, before the war. The doors can be found behind the house just before the cul-de-sac in the northeast part of Sanctuary Hills, next to wild mutfruit plants.
The game files (cell name is "SanctuaryBasementJahani") suggest that this man-made underground shelter was owned or constructed by Jahani and by association he occupied the attached house. The wild mutfruit plants are purposely planted next to the cellar entrance at the rear of the house. There is a store of military ammo inside and a duffle bag cache hidden on the roof, along with the stocked shelter.
In the pre-War introduction stage, the root cellar is not present.
The containers in the root cellar are safe to use for storage and do not respawn. The settlers in Sanctuary will not remove anything stored there, as they cannot travel through cell loading doors.