Other lakes are covered with ice, either permanently or seasonally. In 2006, it was discovered that at least some of the subglacial lakes are connected by subglacial waterways. Over some lakes, the ice rose by several meters during a period of 1–2 years, and then suddenly dropped over a few months to its previous level. These lakes fill with water from an upstream source, while the downstream outlet is blocked. Once sufficient pressure builds up, the outlet is forced open and the water rapidly flows out. One of these is Lake Whillans that lies under 800 m of ice in West Antarctica. It contains a variety of bacteria and archaea in its waters and sediments. Downstream is the ocean under the Ross Ice Shelf. Crustaceans, jellyfish, and fish were found living under the ice shelf. Subglacial and surface lakes exhibit a great deal of species diversity.