North Hall is one of the oldest buildings at the High School. It's a two story building with a basement, consisting of classes and offices on the first floor and more classrooms on the upper floor. It is representative of the late 1800s and early 1900s architecture that comprises the older parts of town - solidly constructed of stone, aesthetically pleasing but still functional. Along with the auditorium, library and science building, it is a defining edifice for the school. It has had a few minor renovations but still retains a lot of vintage elements - radiators in the classrooms for heating, hardwood floors, door locks with keyholes, etc.
The basement is used for records storage and consists of a short hallway with two rooms. The larger of the two rooms contains jumbled stacks of old school desks. A door - welded closed - is located at the far end of the room, behind the desks. Officially, the door leads to a section of the basement that was used during World War II by the Army, who maintained a training garrison on campus (which, at the time, was actually a college). The area is considered off-limits due to structural concerns. It's thought by many, and backed up by historical records, that the area is a much larger sub-basement connecting to other buildings on the campus. It's known that there are also sub-basement entrances in the library and science building.
North Hall has more than its share of unusual occurrences - odd sounds, electrical malfunctions, random air currents, etc. All pretty standard fare for an older building. Students over the years built up quite a bit of lore around the happenings within the building and the school in general. Beyond typical stories about the ghosts of students that have died on the campus, there are a number of legends regarding the door in the North Hall basement. The most persistent are students who claim to have heard knocking, banging or rattling coming from the other side of the door - one story goes that if you knock on the door, something on the other side will knock back. Another is that the door was welded shut after a group of students were found in a state of catatonia just inside the door.
Exactly what is behind the door depends on the nature of the campaign. In conspiracy-oriented campaigns, the rooms behind the door might hold information needed to solve some other mystery. Older faculty or staff may be aware of the content of the rooms, or might possibly even be actively trying to keep it a secret. In a horror game, the rooms beyond might harbor some supernatural threat - spirits, a demon, vampires, whatever works best for the campaign. In survival-horror game, the PCs might even be trapped or held captive within the sub-basement - possibly by whoever originally built it.
A connection to the Army presence on the campus holds quite a few possibilities. There had to be a good reason that they would set up some kind of operation underneath the campus, and using the school as a training garrison is questionable (and probably full of a number of holes). Perhaps it's tied to the location of the school, and what was underneath was already there. The Army operation was to unearth or otherwise utilize whatever could be found underground. It may have been a simple retrieval operation, and whatever is left only holds clues. Experimentation might have gone on as well, and depending on who or what the subjects were, they might still be around.