What was significant about 1968 in International Naval History?

In 1968, 4 countries lost submarines to unexplained disasters.

USS Scorpion (SSN-589) was a Skipjack-class nuclear submarine of the United States Navy and the sixth vessel of the U.S. Navy to carry that name. Scorpion was lost on 22 May 1968, with 99 crewmen dying in the incident. USS Scorpion is one of two nuclear submarines the U.S. Navy has lost.

It was one of four mysterious submarine disappearances in 1968;

The others being:

The Israeli submarine INS Dakar.

The French submarine Minerve.

The Soviet submarine K-129.

Not since WWII have so many submarines been lost in a single year.

If you go to the source, you will find all four incidents listed in the search results. Individual links will take you to the stories about each submarine.