the success of the operation had been largely attributable to the 455 tons of high explosives that the british and anzac tunnellers had quietly buried beneath the german lines over the period of the previous year ; when the detonation had come at 3.10 am it had produced the greatest and loudest man-made explosion ever heard ; so terrible was the noise that it had caused panic in the city of lille some 25 miles away ; in switzerland it had registered on seismographs as an earthquake or a volcanic eruption .