Of the 25 islets listed in 1600, only a dozen remain today. Some disappeared as a result of tidal waves or merged with others. Because the islands are not protected by dykes they are frequently flooded - when this happens only man-made hillocks known as warften, on which the houses are built, remain visible. The most important islands are Langeneß, Oland (joined to the mainland) and Hooge, known as "Queen of the Halligen".