The Hammersee (lake Hammer) is a fresh water lake in the western third of the island.
In the 17th and 18th century Juist was cut in two by several storm tides.
Around the year 1770 people started to close the 2 km wide burst at the southern side
by a dune dike. As recently as 1928 the northern side was repaired. By that time,
the water in the lake had turned from saltwater to freshwater.