Just off the northwest coast of Germany, in the North Frisian Wadden Sea, are a series of ten extremely low-lying islets known as Halligen or Hallig islands. The Hallig Islands rise only a few feet above sea level, so every winter when the tidal flats of the Wadden Sea gets inundated, the islands go under water. In the Middle Ages, there were a great number of Halligen, but with rising sea level several of these uninhabited islets on the Wadden Sea have vanished beneath the waves for ever.

Barely known outside the region, the Halligen are sparsely populated. Each house here is built on top of man-made mounds called warft that keep the structures safe above the waterline. During these periodic flooding events known as landunter, each warft becomes its own tiny island, like the one in the image below.

Hallig Habel, the smallest Hallig in the German Wadden Sea, is a bird sanctuary. The island is uninhabited but has a small house that hosts an ornithological observatory during the summertime. In this photo the island is partially flooded by high tide. Photo credit: Hans Joachim Kürtz/Tumblr