In 2007 a magnetometer survey was carried out at the ring fort Gråborg on the Swedish island of Öland. The results were interpreted as indicating the remains of over 60 buildings, several roads, wells and a large moat outside the fort's northwestern gate. In 2011 these interpretations were severely criticised, and it was suggested that the moatlike anomaly in the geophysical data had actually been caused by a lightning strike. It was also suggested that none of the other interpreted features were actually supported by the presented magnetometry data.

This paper presents the results of a groundpenetrating radar (GPR) survey of the same area. The GPR data were collected in 2014 using themultiantenna Malå Imaging Radar Array (MIRA) system, covering an area of approximately 3.8 ha. The results show that the ground inside and outside the fort's walls is heavily disturbed by farming. Most of the underground features visible in the data can be interpreted as drainage ditches and power cables, but a fewlinear features are identified as being of possible archaeological interest. When comparing the radar data to the buildings, roads andwells suggested in the magnetometry interpretation, no apparent correlation can be established. There is furthermore no sign of any moat in the suggested area. The GPR results therefore support the idea that this moatlike feature is indeed the remains of a lightning strike.