Report - Llanberis RAF Reserve Depot (Bomb Store) - July '11

On 18th August 1939, the Air Ministry sought approval to acquire the disused Glynrhonwy Isaf slate quarry which had closed in 1930; the quarry, near Llanberis in North Wales, was deemed suitable for the storage of 18,000 tons of bombs.

Part of the site resembles eight parallel railway tunnels opening out into a vast concrete tank about 100 metres by 60, with walls 12 metres high. The area was a cut-and-cover construction, formed in the bed of a large slate quarry. There were two levels, and the site was very compact, albeit large. A similar construction was used at Harpur Hill - but never again.

The ceiling of the lower levels forms the floor of the upper levels, which have an arched roof, covered with 8 metres depth of slate waste. One of the galleries is wider and slightly higher than the others, and has a single track railway line running into it the full length, with a wide platform on the North West side and a narrow one the other. An entire train of railway wagons could be brought in for loading and unloading. Such a train, of 27 wagons according to McCamley, was inside of 25 January 1942 when the roof of half the space collapsed, burying the wagons and blocking the only goods exits but not exploding. At the time, 14,000 tons of munitions were stored there, all suddenly inaccessible. Over the next nine months most of the bombs were recovered through the back entrance, which was an adit to another slate quarry. This was a seventy-foot deep pit, and the bombs had to be lifted out of the pit. An inspection at Harpur Hill showed signs of weakness, and much of the overburden was hastily removed. Since the level of the overburden at Llanberis today is not level with the top of the quarry pit, this may also have been done at Llanberis over the uncollapsed part.

[...]

Royal Navy divers were co-opted to investigate the contents of a large lake in one of the pits as it was suspected that it might contain some explosive items. The divers reported that the bed of the lake was littered with explosive items including a number of large bombs. Subsequently, over 20,000,000 gallons (90,920,000 liters) of water and sludge were pumped out. By April 1973 the lake was emptied revealing everyone's worst fears—it took a further two years of hard labour to recover and dispose of the explosive items revealed. Fortunately, this pit was one of those to which 38 Engineer Regiment, RE, had constructed a road, otherwise the task would have been impossible.