Acoustics expert Professor Steven Cox made the discovery near Invergordon when he climbed into the giant underground container, which is twice the length of a football pitch, 30ft wide and more than 44ft high, and began singing and shouting.
The echoes lasted 75 seconds, one minute longer than the previous record held by Hamilton Mausoleum, South Lanarkshire.
Mr Cox said: "Never before had I heard such a rush of echoes and reverberation. I was like a toddler sitting at a piano for the first time, thrashing the ivories to see what sounds would come out. Reluctantly, after a few minutes I stopped playing with the acoustics and started preparing for my measurements."
Archaeological investigator Allan Kilpatrick fired a pistol loaded with blanks about a third of the way into the storage tank while Professor Cox, of Salford University, Manchester, recorded the response picked up by the microphones about a third of the way from the far end.
Listen to the echo by clicking on the icon below
"My initial reaction was disbelief; the reverberation times were just too long," said Mr Cox, but the tests verified that the pair had discovered the world's longest echo.
The tank is in an 80-year-old oil storage complex at Inchindown, near Invergordon. The reservoirs once supplied the naval anchorage in the Cromarty Firth at the bottom of the hill.
The tanks were constructed in great secrecy amid concerns about the strengthening of Germany's armed forces during the 1930s and the threat posed by long-range bombers, which is why they were dug deep into the hillside. The vast complex took three years to complete.
Mr Cox had to enter the tank through one of the 18in diameter oil pipes because there are no doors. The tank could hold 25.5 million litres of fuel and has walls 18in thick.
Mr Cox set out to find the world's "most reverberant" spaces while researching his new book, Sonic Wonderland: A Scientific Odyssey of Sound.
The 1970 Guinness Book Of Records has the last claim for the longest echo. When the solid-bronze doors of the Hamilton Mausoleum slammed shut, it took 15 seconds for the sound to die away due to the building's high stone vault. It compared to just over nine seconds at St Paul's Cathedral, London.
The mausoleum was completed in 1858 as the resting place for the Duke Of Hamilton's family. The Duke was interred in an Egyptian sarcophagus, on a black marble slab in the main chapel, while 17 of his ancestors were interred in the crypt below. However the coffins had to be removed later and buried in a local cemetery due to subsidence and flooding.
An average living room will record an echo of 0.4 seconds, according to research, or 1.2 seconds for an opera house.
Other sound records listed in the Guinness Book Of Records include the loudest purr of a domestic cat, at 67.7 decibels, and the loudest clap at 113 decibels.