The illegal ‘hidden green lagoon’ that lures people despite warnings about what lies beneath

A hiker exploring a notorious quarry in Gwynedd found evidence that the dangerous waters are still used for swimming. Offenders risk prosecution at the disused quarry near Llanberis and numerous warnings have been issued over the years about the dangers of the site.

Upper Glyn Rhonwy is surrounded by a 1.8 meter high fence, but some visitors continue to ignore warning notices posted on it. In 2018, a major rockfall occurred at the quarry and a lake inside contains underwater hazards such as old mine waste. There are also rumors of nerve gas bombs being dumped, despite assurances from the Ministry of Defense that the site has been thoroughly cleared.

Despite the risks, swimmers continue to enter the site. There is a public footpath nearby, parts of the old fallen fencing have yet to be replaced and the rugged beauty of the quarry is a natural attraction for the adventurous. “People swam in it today,” said a recent visitor online. “It’s deep and probably quite cold!”

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Evidence of routine access can be seen in the quarry’s graffiti, although this is not as prevalent as the ‘art in the dark’ in the lower pit. Plans to build a 100MW pumped storage project, using Glyn Rhonwy’s two wells, were approved in 2017, sparking renewed interest among visitors, amid fears it would not be accessible for much longer. Snowdonia Pumped Hydro was recently granted a further two-year extension, until March 29, 2026, when work is due to begin.

Climbers drawn to a quarry with routes like Cracking Up and Clippopotumus note that there have been few reports of access problems for years. Wild swimming bloggers also extol the virtues of Glyn Rhonwy, describing its “good quality” water with a shale “beach” area.

YouTube videos show people jumping from a cliff into the quarry’s lagoon, while Google Earth images show swimmers in wetsuits preparing to dive in. Warnings that the water “may contain harmful chemicals” have not deterred people due to the promise of relative seclusion in a place often described online as “beautiful” and a “hidden lagoon”. Seen from above, the water appears deep and seductively green.

Residents have mixed opinions about the use of the site, with some disapproving of the waste left behind more than the swimming itself. Others see the warning signs as an overreaction. “Shocking,” one man said. “Young people enjoy!” North Wales Live has a WhatsApp community group where you can get the latest stories straight to your phone

Visits are not productive given their illegal nature, although for some this is part of the appeal. The quarry tends to become busier in summer, although Cyngor Gwynedd says the waters can remain ‘dangerously cold’ all year round. Get the latest Gwynedd news by signing up to our newsletter, sent every Tuesday

A visitor from Cornwall wrote online that there are good reasons why swimmers should be wary. “I’ve swum there a lot in the past,” he says. “But I also saw a video of a boy who was almost hit by several tons of falling rocks while he was in the water. You understand why the community is not enthusiastic and why people are nervous, especially when children are involved.”

There were some concerns about the use of the lower quarry as a bomb storage facility during the Second World War, when the area was known as RAF Llanberis. The site was later used for the disposal of thousands of tons of ammunition, including bombs, bullets and incendiaries.

As part of the post-war Operation Dismal, 14,000 tons of bombs filled with Tabun nerve agent were temporarily stored there after being removed from Germany. Shortly afterwards these were moved to Llandwrog and later taken out to sea.

A six-year cleanup of Glyn Rhonwy was carried out by the RAF in the 1970s. Following a Freedom of Information (FoI) request, it emerged that several mustard gas bombs had been found there at the time.

Another FoI request submitted by campaigners, asking for details of Glyn Rhonwy’s stockpiled Tabun bombs, hit a brick wall. Although the existence of a contemporary report was confirmed, the application was rejected on public interest grounds.

The Ministry of Defense explained: “Releasing the information would enable malicious individuals or organizations to act against the national interest.” However, the Ministry of Defense told the BBC that, following a thorough investigation in 1975, the site was “assessed as being free of ammunition”.

Cyngor Gwynedd issues almost annual reminders about the access bans to the site and the dangers of swimming there. Local police teams have also pledged to continue patrolling the quarry, warning that the area is “prone to rock falls and landslides”. Officers say people found in the quarry could face charges.

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