Rogue waves (also known as freak waves, monster waves, episodic waves, killer waves, extreme waves, and abnormal waves) are unusually large, unpredictable and suddenly appearing surface waves that can be extremely dangerous to ships, even to large ones.They are distinct from tsunamis, which are caused by the displacement of water due to other phenomena (such as earthquakes) and are often almost unnoticeable in deep waters. A rogue wave appearing at the shore is sometimes referred to as a sneaker wave.