In 1958, the United States conducted a series of nuclear tests at the Pacific Proving Grounds in the Marshall Islands as part of Operation Hardtrack. In some recently remastered footage of the Wahoo blast (a 9-kiloton underwater test), there are three distinct shock waves generated by the bomb.
The first shock wave (12-second mark) was obviously caused by the initial explosion. The 2nd (at 20 seconds) was probably the cavitation shock wave created by the seawater rushing in to fill the massive void left by the blast.
But then, to my surprise, there is a 3rd, smaller shock wave at 42s in the video. What caused this?