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.

History
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 m long, 9 m wide, with arched roofs 13.5 m 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 build to be a bomb-proof supply of Furnace Fuel Oil to the Royal Navy's base at Invergordon.

In 2014, the tanks were designated as a Category A listed building.

Reverberation record
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 Royal Commission on the Ancient and Historical Monuments of Scotland's Allan Kilpatrick 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.