Water Tentacle first appeared in the episode "The Awakening" when Will Benjamin accidentally witnessed a powerful mystical force on Mako Island. It is believed that the water tentacle and the large waterfall that covered the wall around the Moon Pool was caused by the Full Moon. It was later revealed that the appearance of the Comet Eva is what activated the water tentacle. And it appeared during full moons, when the moon's light would reach the Moon Pool, which was required for the magic, to show the danger of the comet, to work.