Cwm Idwal is a hanging valley in the Glyderau range of mountains in northern Snowdonia, the national park in the mountainous region of North Wales. Its main interest is to hill walkers and rock climbers, but it is also of interest to
geologists and naturalists, given its combination of altitude (relatively high in UK terms), aspect (north-facing) and terrain (mountainous and rocky). In a 2005 poll conducted by Radio Times, Cwm Idwal was ranked the 7th greatest natural wonder in Britain.
That's how Wickipedia describe it - and having been there myself I would agree! Despite living in South Wales, my knowledge and experience of North Wales is fairly limited, but I'll definitely be back for more!