What caused the 3rd shock wave in this nuclear explosion?

Question

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?

Answer 1

The most likely cause of the second (big) explosive event was venting the hot vapor, entrained superheated water, and vaporized sand and coral debris out of the underwater expansion bubble and into the atmosphere, where it then freely expanded. The third (small) explosion was probably the rebound shock from the collapse and re-expansion of the explosion bubble.