The Aquariums are a defensive mechanism that is hidden within Yggdrasil; there are three of them total. They protect Riviera's guardian, Ursula. Aquariums are large diamond-shaped structures that apparently hold water inside; they are fragile, and thus require the protection of the Cefiro. During the events of Chapter 6 - The Promised Land, both Ledah and Malice infiltrated Yggdrasil in order to destroy these precious artifacts, each with their own motives. Ein and the sprites entered Yggdrasil in pursuit of Ledah, but ultimately they were unable to prevent the Grim Angels from smashing each of them.