VISITORS stepped back in time to explore a mysterious hidden Second World War weapons facility.

Gates at the former top secret weapons site were opened for the second open day at Rhydymwyn’s Valley Works, where some of the initial designs for the atomic bomb were drawn up.

The underground network also stored mustard gas shells and were once earmarked to store 4,700 tonnes of gold from the Cold War – the entire UK gold reserve from the Bank of England – in the event of a Soviet attack.

The open day gave visitors a glimpse inside the tunnels with local experts providing guided walks and accompanied tours around the site.

Colin Barber, secretary of the Rhydymwyn Valley History Society, said the event had exceeded all expectations with about 500 visitors exploring the tunnels.

“So many people don’t know they can visit the site all year round, so the open day helps get that across,” he said.

“We fought long and hard to get the site open to visitors and lots of people came from all over the county, many more than we anticipated.

“We want to let people know what is here, its importance, and that it is also such a beautiful place.”

Developed in 1939 at a cost of more than £546,000, the site started producing weapons-grade uranium with atomic potential in 1942 and played a vital part in the research undertaken prior to the Manhattan Project, ultimately leading to the Hiroshima atom bomb.

From 1941-45 it produced mustard gas shells for the war, which were not deployed by the British during the conflict, and until 1980 it served as a buffer depot site storing food and supplies in case of emergency.

At its height the complex consisted of more than 120 buildings, 94,000sq ft of tunnels and seven miles of secure fencing.

Some of the brick buildings were given protected status in 2008 by the Welsh heritage society, CADW.

Also on show was a display of vintage cars, bikes and military vehicles which gave both adults and youngsters the opportunity to get a feel for the motoring wonders of yesteryear.

They took the chance to get into the cockpit of a fighter aircraft, the seats of armed vehicles, Jeeps and see a Green Goddess fire engine as part of the array of vehicles exhibited.

To arrange a visit to the tunnels visit ryhdymwyn or email