The Mumiengrotte is a peculiarity that sits in the Mariendal valley park west of the city centre — a low cave entrance in an artificial grotto, inside which an ancient stone sarcophagus rests on a plinth. The origin of the sarcophagus is uncertain, but it is generally believed to be of ancient Egyptian or Levantine origin, brought back to Flensburg by a wealthy merchant in the early 1800s.
The grotto is unlocked and open at all times, sitting in a public park with no signage or interpretation. This is part of its appeal: finding it requires some knowledge and a short walk through a pleasant wooded valley. It is genuinely one of the stranger things in this part of Germany, and it costs nothing.