The bomb squad was called in to carry out a controlled explosion after an old piece of ammunition was found.
Passers by say they could hear the explosion more than a mile away from the site where police experts were carrying out their work.
Officers were called out to the scene, a location near Crofton, west of Thursby, shortly before 10am this morning, when the ammunition was found.
Eyewitness Frank Chalmers said: "I heard the bang at about 2.15pm. It was very loud and you could hear the shockwaves about a mile away."
At least three police vans and other officers could be seen at the incident.
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