Pretty incredible if you ask me. This is from the national choral festival they put on in Estonia every couple of years. The dome behind the choir is a choir shell specifically designed for this purpose. Choral singing is fundamental to the Estonian national identity. Sounds like my kind of place!
Here are the opening ceremonies for the 2009 festival. Skip to 4:30 to hear the opening song (and the conductor giving pitches). This video gives a better idea of the scope and size of the event.