"A basement parking garage is interesting. But, would people at ground level still be able to here the music? Also, if it's at sea level can you have a basement?"
The Wshburn Tunnel in Pasadena, TX goes under the Houston Ship Channel, so I imagine anything is possible.
People at ground level may not be able to hear the music, but then that would be better for the ravers, wouldn't it?
You mean like the tunnel under the English channel that connects France with England? ........ Amazing.