A freak "boom" in the middle of the night set off car alarms and woke residents.
Student Jason North, 23, was one of several people to be disturbed by the noise heard along the south coast in the Peacehaven area at 1am today.
Mr North, from Telscombe Cliffs, said: "It was like a sonic boom. It sounded a long way off but it made enough force to make the car alarms go off."
Residents in Saltdean and Peacehaven also reported hearing the noise.
Police, fire and coast guard services were at a loss to explain the incident. They said there had been no reports of any explosions on land or at sea.
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