Street art originated in Philadelphia in the 1960s before expanding to New York City in the 1970s. Artists began painting subway cars and building walls, transforming vandalism into an art form. Famous street artists include Banksy, whose politically-charged work remains anonymous, along with David Choe, Retna, and Escif. In 2012, Banksy unveiled pieces in London criticizing issues like child labor ahead of the Olympics. Escif is renowned for a giant light switch mural in Poland. The most iconic work of street art may be the graffiti-covered Berlin Wall before its demolition. While some artists work anonymously, Banksy and Choe have been paid hundreds of thousands for their pieces.