The 20th century was one of ballet's experimental phases. Russian choreographers Sergei Diaghilev and Michel Fokine began going beyond classical ballet. Eventually, Diaghilev, in collaboration with Igor Stravinsky, developed The Rite of Spring, a ballet that broke the boundaries of ballet. The music was dissonant, the story of human sacrifice was original and somewhat controversial, and the movements were much different than the standard of ballet back then. George Balanchine, a Russian choreographer, also developed neo-classical ballet and is considered the father of "plotless" ballet, ballet without a story and meant to express human emotions.