No. 96: Nov-Dec 1994
"Scientists based on the central California coast are trying to identify the origin of a mysterious underwater sound that disturbed surfers and divers for three weeks -- and then just as mysteriously disappeared.
"The sound, made up of thumps occurring at 10-second intervals, was compared by one diver to five or six giant bongo drums going off simultaneously. Most experts have concluded that it was of human origin."
As usual in such cases, no governmental or military sources knew anything about the thumps.
(Shurkin, Joel N.; "Underwater Thumps Baffle Ocean Scientists," Nature, 371: 274, 1994.)