Inchindown oil tanks
The Inchindown oil tanks are a disused underground oil depot in Invergordon, Ross-shire, Scotland. The tanks hold the record for the longest reverberation in any man-made structure, surpassing the Hamilton Mausoleum in 2014.
The site was officially called "Inchindown, Royal Navy Fuel Tanks" and also known as the "Invergordon Oil Fuel Depot". The complex consists of six tanks: five were 237 metres (778 ft) long, 9 metres (30 ft) wide, with arched roofs 13.5 metres (44 ft) high; a smaller sixth tank was of the same height and breadth but less long. Work on the tanks began in 1938 and was completed in 1941. They were built to be a bomb-proof supply of Furnace Fuel Oil to the Royal Navy's base at Invergordon.
In 2009 guided tours of the tanks were offered by Forestry Commission Scotland. After the tours were mentioned on BBC television programme The One Show, professor of acoustic engineering at the University of Salford, Trevor Cox was motivated to perform reverberation tests in the underground tanks. In 2014 Allan Kilpatrick from the Royal Commission on the Ancient and Historical Monuments of Scotland fired a pistol blank inside one of the tanks. The sound was recorded by Cox, and is reported to have reverberated for 112 seconds at 125 Hertz, 30 seconds at mid frequency, and 75 seconds broadband. 
|Wikimedia Commons has media related to Inchindown oil tanks.|
- McKenzie, Steven (8 August 2011). "Return to Highlands' Inchindown secret tunnels". BBC Scotland. British Broadcasting Corporation. Retrieved 18 July 2014.
- Adam Sherwin (16 January 2014). "Now we've heard it all: Acoustic scientists shatter the world record for longest ever echo". The Independent. Independent Print Limited. Retrieved 18 July 2014.
- Historic Environment Scotland. "Underground Fuel Reservoir, Inchindown, Invergordon (Category A) (LB52317)". Retrieved 28 March 2019.
- "First glimpse of world war depot". BBC News Scotland. British Broadcasting Corporation. 22 September 2009. Retrieved 19 July 2014.
- Cox, Trevor. The Sound Book. W.W. Norton & Company, 2014, p. 57.
- "New world record for longest echo set near Invergordon". BBC News Scotland. British Broadcasting Corporation. 16 January 2014. Retrieved 24 July 2014.