Echo tube
This is one end of a very long galvanized steel tube. If you clap into it, the echo comes back as a sequence of pinging sounds.
Taken Monday, July 02, 2001, 10:33:21am. Original image size: 1600x1200, 970kb
Technical details: Olympus C700UZ, 1/30s @ F3.5, ISO 320, 171mm