Royal Air Force Llanberis was opened as an explosive storage unit in May 1941, close to the Snowdonian village of that name in north Wales. The site occupied a large complex of disused slate quarries and interlinking tunnels. Between 1941 and July 1956 when the unit closed it had been used as a bomb store, a demolition area and a burning pit.
Overhead protection was given by forty feet of broken slate. In response to pressure from the treasury efforts were made to cheapen and accelerate the construction of Llanberis, but unfortunately the cost cutting had disastrous consequences only six months after the depot was opened.
On 25th January 1942, two-thirds of the structure collapsed within seconds under the weight of the overlaying backfill, completely engulfing a train of twenty seven wagons which was in the process of unloading. The collapse buried over 14,000 tons of bombs which at the time represented 14% of the total RAF stock. A court of inquiry concluded that faulty design was the principal cause of the failure; cracks were noticed in the structure as the building neared completion but these were attributed to minor defects rather than to a major and fatal miscalculation.