Article: In June, 1998 Sammy Sosa exploded into the pursuit of Roger Maris' home run record. Sosa had 13 home runs entering the month, representing less than half of Mark McGwire's total. Sosa had his first of four multi-home run games that month on June 1, and went on to break Rudy York's record with 20 home runs in the month, a record that still stands. By the end of his historic month, the outfielder's 33 home runs tied him with Ken Griffey, Jr. and left him only four behind McGwire's 37. Sosa finished with 66 and won the NL MVP Award.

Question: When did Sammy Sosa explode into pursuit of Roger Maris' home run record?
1998