Looking for a fic

Hey all I’m looking for a fic I read ages and ages ago. I can’t for the life of me remember the name of it. I believe it takes place in the first couple years of Hogwarts. Harry is in potion’s class when there is an accident and it causes the dungeons to cut off from the rest of the castle. The house elves are not even able to get in. Snape has to try and keep all of the classmates alive through feeding them potion ingredients. Harry gives his away while they are down there Snape discovers Harry’s abuse at the hands of Dursley‘s and there is like a countdown with how long they think they’re going to survive