The Great Lake, often referred to as the Black Lake, is a vast, deep body of water located on the Hogwarts Grounds. Its surface can vary from being as smooth as black glass to choppy and icy grey, depending on the weather. The lake is a fundamental part of the Hogwarts landscape and has been present since the school's founding. The lake is teeming with magical life. Its most famous inhabitant is a Giant Squid, which is largely placid and has been known to interact harmlessly with students, even saving Dennis Creevey after he fell from a boat. A reclusive and territorial colony of Merpeople inhabits a village in its depths, complete with crude stone dwellings and a central statue. These Merpeople are wary of wizards and witches, possessing a harsh, screeching language when above water but a more comprehensible one beneath the surface. The shallows of the lake are also infested with aggressive water demons known as Grindylows. For centuries, the Great Lake has been the final leg of the journey for new students. First-years traditionally cross the lake in a fleet of small boats, guided by the Groundskeeper, which provides them with their first breathtaking view of Hogwarts Castle. During the 1994-1995 school year, the lake also served as the mooring point for the spectacular, skeletal ship from the Durmstrang Institute.
The Great Lake is the setting for several significant events throughout the series.