The Dungeons
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Places of Interest
  1. Snape’s Potions classroom and office
    2. Slytherin common room and dormitories
    3. Assorted chambers and corridors, leading to the 
        Philosopher’s Stone
    4. Dungeon large enough to host a Deathday party
    5. Dungeon 5 (so, presumably, dungeons 1-4 as well)
    6. Snape’s rooms

Events
       Most of what is unpleasant at Hogwarts--with the exception of any possible portion of Professor Trelawney’s love life--takes place “down in the dungeons”. These include Potions classes, Occlumency lessons, Deathday parties, and life-or-death struggles against Voldemort. I have included Snape’s rooms in the dungeon level as well, because he seems to enjoy it there so much.

Secrets and Spells
       Hiss the word “open” to the sink in Moaning Myrtle’s bathroom and a pipe will widen in the wall. It shoots downwards, ending in the Chamber of Secrets. This legendary room is described as being somewhere “below the lake” amidst a pile of rat skeletons and a Basilisk (deceased).