The cellars were accessed by a door on the ground floor, between the sixth-form room and the headmaster's study. Through the door were stairs down to a series of small, dungeon-like rooms, dark and dirty and barely lit (by a few naked bulbs, I think); if you persevered, you came through at last to the exit, emerging up some stairs into the open air just under the library windows.
The cellars were used to store boxes belonging to the boys. I don't remember what they were supposed to contain, nor what they actually contained; but each boy had a locked box sitting down there, like a smallish trunk. I suppose that's why we were allowed to go down there; otherwise, the cellars would probably have been kept locked.