'Mystery' in the sky as video captures helicopter carrying giant metal ring over city

Video shows a helicopter flying over a UK city, while carrying a large metal circle. In the video, shared to social media, the helicopter - with a rope attached - transports the wide metal structure across the sky.

The helicopter, which was doing an electromagnetic survey, was spotted in Truro, Cornwall.

This system is called HeliEM or Heliborne Electromagnetic Survey. A current is passed through a loop, creating an electromagnetic field. In response to this field, currents are induced underground, special sensors measure the resulting secondary field, which allows an understanding of the structure of underground rocks.