A large lake in the Hogwarts grounds that was home to many creatures including the giant squid (who was friendlier than you might initially think), Grindylows and even merpeople. Many students liked to hang out by the lake on days when the weather was pleasant. It was also the location of the second Triwizard Tournament task, where Harry and the other champions had to dive in and spend up to an hour in its depths among the beings that called it home.