Here’s an interesting bit of news from the world of sound: a decommissioned underground oil storage depot in the north of Scotland has been found to have the longest echo of any man-made structure anywhere in the world. Echoes in the Inchindown tunnels have been found to last for as long as 112 seconds1 with the broadband reverberation2 time for the tunnel lasting for 75 seconds.
Below you can hear an echo from Inchindown. The original sound is a pistol being fired a third of the way into the tunnel, and the microphone is the same distance from the far end.
Amusingly3, the last claim for the longest echo in a man-made structure was supposedly also from Scotland: in 1970 the echo of the solid bronze doors of the Hamilton Mausoleum slamming shut was found to last for 15 seconds.