Bomb disposal experts are on their way to the Helford River after a diver found what could be an unexploded device.
Mark Milburn of Atlantic Scuba first found the device about half a mile off shore in the mouth of the river two weeks ago, but was only able to take photos to send to the experts yesterday.
And they believe it could be a parachute mine.
This afternoon they are going to assess the device and are expected to take it further out in the river to conduct a controlled explosion.
Mr Milburn said: "At first I thought it was a barrel, but on closer inspection I realised it was something else."