During the 1990s, Diamond produced six studio albums. He covered many classics from the movies and from famous Brill Building-era songwriters. He also released two Christmas albums, the first of which peaked at No. 8 on Billboard's Album chart. Diamond also recorded two albums of mostly new material during this period. In 1992, he performed for President George H.W. Bush's final Christmas in Washington NBC special. In 1993, Diamond opened the Mark of the Quad Cities (now the iWireless Center) with two shows on May 27 and 28 to a crowd of 27,000-plus.  The 1990s saw a resurgence in Diamond's popularity. "Sweet Caroline" became a popular sing-along at sporting events. It was used at Boston College football and basketball games. College sporting events in other states also played it, and it was even played at sports events in other countries, such as a Hong Kong Sevens rugby tournament or a soccer match in Northern Ireland. Most notably, it became the theme song of Red Sox Nation, the fans of the Boston Red Sox. The song also came to be played during the 8th inning of every New York Mets home game.  The New York Rangers also adapted it as their own, and played it whenever they were winning at the end of the 3rd period of their games. The Pitt Panthers football team also played it after the third quarter of all home games, with the crowd cheering, "Let's go Pitt". The Carolina Panthers played it at the end of every home game they won. The Davidson College pep band likewise played it at every Davidson Wildcats men's basketball home game, in the second half.

Answer this question "What happened in the 90s?" by extracting the answer from the text above.
The 1990s saw a resurgence in Diamond's popularity. "Sweet Caroline" became a popular sing-along at sporting events.