The Great Lake

The Great Lake is a large body of water located to the south of the castle. It is about half a mile in diameter and the Hogwarts plumbing network drains into it. It is home to a giant squid and a colony of Merpeople. The lake was also used as the site of the Second Task of the Triwizard Tournament. The Slytherin common room is beneath the lake, accessed through an entrance in the school dungeons. The Chamber of Secrets is also thought to be beneath the lake.