At a glance
Llyn Cwm Silyn (LL54 6BN) — a remote mountain lake in a quiet cwm below the Nantlle Ridge. 3-mile walk in from Nantlle village. Wild swimming for experienced swimmers only (very cold · remote · 3 miles from help). Free. Car recommended. Best July–August.
About Llyn Cwm Silyn
Llyn Cwm Silyn lies in a hidden cwm below the Nantlle Ridge — a 3-mile walk from the village of Nantlle through a landscape shaped by centuries of slate quarrying. The approach follows old quarry tracks into the upper cwm, where the lake occupies a bowl below the steep southern face of Y Garn. The lake is rarely visited: the Nantlle Valley is one of the less-frequented parts of Snowdonia, and the walk to the lake is long enough to deter casual visitors. On a summer weekday the cwm may be entirely empty.
The remoteness is part of the appeal for serious wild swimmers. The water is cold, clear, and fed from high mountain drainage — the quarry slate that makes up the cwm walls doesn't release minerals that would colour or contaminate the water. The setting is dramatic: steep rocky cwm walls on three sides, the ridge crest above, and the Llŷn Peninsula visible to the west on clear days. This is wild swimming at the quieter, more committed end of the spectrum — not suitable for first-timers, but deeply rewarding for those who make the effort.
Safety information
Wild swimming carries serious risks. Never swim alone. Tell someone where you're going and when you'll be back. Enter the water gradually to avoid cold-water shock. Know your exit point before you start. Wear a tow float and consider a wetsuit even in summer — Welsh lake and sea temperatures rarely exceed 16°C.
In an emergency call 999 and ask for the Coastguard (sea) or Police / Mountain Rescue (lakes & rivers). Nearest A&E: Ysbyty Gwynedd, Penrhosgarnedd, Bangor LL57 2PW. Tel: 01248 384 384.
Find it on the map
Frequently asked questions
The standard approach to Llyn Cwm Silyn starts from Nantlle village (postcode LL54 6BN), in the Nantlle Valley south of Caernarfon. From the village, a track and then path head south-east into the cwm, climbing approximately 300 m over 3 miles to reach the lake. The path follows the line of the old quarry tramways that served the Dorothea slate quarry — the flooded remains of which are visible in Nantlle village itself. The walk takes approximately 1.5 hours each way in good conditions. There is limited parking in Nantlle village; a car is the most practical way to reach the start.
No — Llyn Cwm Silyn is a remote mountain lake suitable for experienced wild swimmers only. The 3-mile walk-in over rough terrain means that any emergency requires a long walk out. The water is extremely cold — fed from snowmelt and high mountain drainage — even in summer. The lake is rarely visited, so there is no passing traffic to assist in an emergency. The standard wild swimming precautions apply with particular force here: never swim alone, tell someone your exact plan including expected return time, enter the water very gradually to avoid cold water shock, and know your exit point before entering. The nearest A&E is Ysbyty Gwynedd, Penrhosgarnedd, Bangor LL57 2PW, tel: 01248 384384.
The Nantlle Valley (Dyffryn Nantlle) was one of the most intensely quarried slate areas in North Wales. The Dorothea, Pen-yr-Orsedd, and Cilgwyn quarries between them produced millions of tonnes of slate in the 19th and early 20th centuries. The village of Nantlle itself partly occupies the infilled area of the Dorothea quarry, and the flooded pit — a vivid turquoise lake — is visible from the road. The quarrying heritage gives the valley a distinctive industrial-romantic character: wild mountains and cold lakes above, the flooded blue pits of abandoned quarries below. The Nantlle Ridge walk above the cwm is one of the finest ridge walks in North Wales, offering outstanding views over the Llŷn Peninsula and Snowdon.
Yes — for experienced hill walkers, a combination is possible. The Nantlle Ridge walk (one of the finest in Snowdonia) runs along the ridge above Cwm Silyn, with Y Garn (634 m) directly above the lake. From the lake, it is possible to climb steeply to the ridge and then walk east along the crest to the main Nantlle Ridge path, descending back to the valley from the eastern end. This creates a long day walk of 8–10 miles and 700+ m of ascent, suitable only for experienced walkers with navigation ability. A simpler option is the out-and-back to the lake for wild swimming only (6 miles, 3–4 hours), without the ridge ascent.
Llyn Cwm Silyn has good water quality — the lake is fed by runoff from the high Nantlle Ridge above and the old quarry workings nearby. The quarry slate doesn't significantly pollute the water (unlike copper or metal mining, slate quarrying doesn't release harmful minerals into water). The lake is cold, clear, and largely unpolluted as the cwm sees very little recreational traffic compared with more accessible Snowdonia lakes. As with all wild swimming in remote mountain areas, conditions vary with rainfall and season. The absence of agricultural land immediately around the cwm (the upper cwm is above the farming limit) means the water is generally clean. Check for any local Natural Resources Wales (NRW) water quality notices before swimming.