The Great Lake, also known as the Black 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. Creatures have made it their home from Merpeople to Grindylows, with even Moaning Myrtle occasionally taking a swim. The most notable is Bartholomew Henry, the Giant Squid. While busy with his kin, it is not uncommon to spot him or possibly receive a helping hand should you slip in after enough callings.

Boats and life vests mainly reserved for the First year's entrance and Graduate's departure still sit by the dock. With Sir McLeod's permission you'll be permitted to use them for a lovely afternoon of sailing. There are also several trees by edge of the lake. A Beech tree in particular with a hollow hole inside is filled with books that have been left there over time. If you take one, be sure to replace one in exchange to keep the system alive.