Complex sentence: Project 949 submarines were the largest cruise missile submarines in service, until the Ohio class SSGN cruise missile submarine converted from SSBN and returned to service on October 15,2007, and the third largest submarines in terms of displacement and length.
Split into simpler sentences: Project 949 submarines were the largest cruise missile submarines in service, until the Ohio class SSGN cruise missile submarine converted from SSBN and returned to service on October 15,2007. They are the third largest submarines in terms of displacement and length.