Shape Note Singing or Sacred Harp singing is a tradition of sacred music that originated in the American South of the United States. The music is notated with different shapes rather than formal music notation, allowing for ease of reading and the inclusion of a larger community. The songs are always performed a cappella and generally sung as loud as possible, with the singers themselves arranged in a hollow square with a leader at the center. This sacred music originated from Protestant Christian music but is much more complex, emphasizing a fugue-like style and emphasizing fourth and fifth intervals. 

This style of music is still very much in effect today, with "sings" occuring all over the USA. All are welcome, regardless of musical experience, and participation can be very cathartic, especially for the leader in the center of the singing square. 

Learn more about Sacred Harp here.
Listen to an example here.