Behind the castle lies the Black Lake. Though it may appear vast and lifeless from quick glances out the castle window, if you spend enough time around here, you will discover it to be teeming with life. Perhaps the most notable resident is Bartholomew Henry, the giant squid. Though rarely seen, it is not uncommon to notice him lazily sunbathing in the warmer months. Other denizens of the deep include a colony of Merpeople, grindylows, and, on occasion, Moaning Myrtle—fresh from the castle, flushed into the Lake by an unaware student.

Students are free to pull up a patch of grass to do some homework, take a nap, or just spend time with friends.