Whomping Willow

The Whomping Willow is a large, magical and violent tree in the fictional world of Harry Potter. One such tree is known to reside on Hogwarts grounds; it's not known whether this is a unique specimen or a member of a species. The Hogwarts tree is rooted to the spot, but will lash out at anyone who comes too close. Planted very young and now likely in its twenties, it's grown to a good height and become quite dangerous. On its early days, students often deliberately came under attack and tried to duck the blows, but this was eventually discontinued as it became too risky. The tree has served as more than an example in aggressive botanics, however. It was planted to enable werewolf Remus Lupin to attend the school. The Willow covers one of the few remaining secret passage into and out of Hogwarts, this one leading into the abandoned and boarded Shrieking Shack in Hogsmeade. Lycanthropy not being a popular trait, Lupin would use this tunnel to spend every full moon in the Shack. As a... full article at wikipedia