http://phys.org/news/2014-01-world-longest-duration-echo-man-made.html
New world record set for longest duration echo in a man-made structure The whispering gallery wave is central to photonic energy concentration in LENR. An echo lasting just under two minutes was created in a large oil tank. The sound wave of a gunshot traveled round the inner tank for a very long time before it lost all its strength.
