Llyn Llydaw (welsh for lake Brittany) is one of the lakes you pass if you go from Pen-y-pas via the Miners track to the foot of Snowdon. This location makes it to one of the most visited lakes in the UK. It is up to 200 feet deep but it contains no fish; it is also considered to be one of the coldest lakes in the UK with 7°C and it is a candidate for an Excalibur lake (a lake from which Excalibur came and eventually returned on the mortal wounding of King Arthur - there are about 10 candidates but due to the location Llyn Llydaw is a quite unlikely one).

The ruins of the old Britannia copper works crushing mill are near Llyn Llydaw, and amongst the remains are the large crushing hammers that were used to extract the valuable ores. Copper ore was taken down to the crushing mill by an aerial ropeway over Llyn Glaslyn; this reduced the distance the copper had to be transported, and avoided the steep climb between the two lakes.