Little-known everyday physics: all underwater bubbles are musical instruments! When stimulated (depth charges, guns, or just by whacking nearby rocks together) they can give out bell-like tones. Clouds of bubbles give out deep bass notes, as when beating meringue or when performing the Hot Chocolate Effect. So, SWIMMING POOL TRICKERY: http://amasci.com/~billb/cgi-bin/instr/instr.html#hot
