The Nant Ffrancon Pass in Snowdonia, North Wales, is the long steady climb of the A5 road between Bethesda, Gwynedd, and Llyn Ogwen in Conwy. The summit at 312 metres is at Pont Wern-gof, about one-third of a mile beyond the eastern end of Llyn Ogwen. From here the road descends through Nant y Benglog to Capel Curig and through to Betws-y-Coed. The A5 is the Holyhead to London trunk road, which was re-engineered by Thomas Telford between 1810 and 1826. The original road through the Nant Ffrancon was constructed by Lord Penrhyn in the late 18th century, and at Capel Curig in 1801 he built a coaching inn, which is now Plas y Brenin, the UK National Mountaineering Centre.
Nant Ffrancon itself is a steep-sided glacial valley dropping to Bethesda between the Glyderau and the Carneddau. The valley starts in Cwm Idwal, carrying water from Llyn-y-Cwn through Twll Du and Llyn Idwal to join the Ogwen Valley below the Ogwen Falls on Afon Ogwen. Unlike Lord Penrhyn's road, which largely followed the valley floor, Telford carved much of his road out of the north-eastern slopes of the Nant Ffrancon, thereby encountering difficulties in construction and future maintenance.