South of Sandager lies the small Helligsø. In 1990, the pump that had drained the area for agriculture for around 50 years was turned off, the 200 m long dike was removed and an inlet to the Flensburg Fjord was dug. This means that the salt water of the fjord now mixes with the groundwater of the lake. The result is the Helligsø, a saltwater lake that is around 3 hectares in size at normal water levels, but can be as large as 7.5 hectares at high water levels. Since the water level in the Baltic Sea and the fjord also changes (albeit lower in normal cases), 2/3 of the lake flows into the fjord at every tide. The groundwater of the lake can therefore always be replenished, so that there is always fresh and nutrient-rich water available. Plants and water birds typical of saltwater live on and in the Helligsø.
Saltwater-typical plants and water birds live on and in the Helligsø. From the bridge that crosses the lake's outlet, you can enjoy a small view of the lake.
Otherwise, the area is a bird sanctuary and should not be entered.