The Army Bomb Disposal Team were tasked to Portrush on Tuesday to detonate a marine flare that washed up on the beach.
The flare was discovered in the afternoon by a member of the public, who carried it from the West Strand beach to the promenade and then dialled 999.
PSNI officers sent for Army experts, who returned the flare to the beach, buried it, and then detonated it in a controlled explosion.
Coleraine Coastguard Station Officer Chris Little, whose team assisted the PSNI in dealing with the situation, said: “Our advice to the public is that if they find anything suspicious on the beach leave it alone. Just dial 999 and ask for the Coastguard.”
The West Strand was closed to the public for several hours while the operation was under way.