Deep underground in the Scottish highlands lies a complex of disused oil storage tanks built during World War II. Located near the town of Invergordon, the Inchindown Oil Tanks were used to cache British stores of furnace fuel oil in bombproof environs, all the while hiding a sonic secret.

In 2014, 12 years after the tanks were decommissioned and emptied, an acoustic engineering professor named Trevor Cox discovered that they were capable of producing the world's longest echo in a manmade structure. 

Professor Cox got a tip from a local that one of the tanks was large enough to create a huge echo. The tank is twice the length of a soccer field, and 44 feet tall. After being lowered into the tank in a pipe (a stunt that left him covered in oil), Cox recorded the sounds of blanks fired from a gun. The sound reverberated for an incredible 112 seconds, easily beating the standing world record for longest echo. Even the tones of regular speech hung in the air for upwards of 30 seconds. 

The vast space of the Inchindown Oil Tanks complex has no known rivals in its capacity for echoing. There is, however, a chance to visit this amazing underground construction on a private tour.