Eryri National Park covers 823 square miles, the fourth largest of the United Kingdom's national parks. It was designated in October 1951 - the third UK national park, after the Peak District and the Lake District established earlier that same year. The boundaries take in central and southern Gwynedd, much of the western Conwy County Borough, and twenty-three miles of coast along Cardigan Bay. Within this area sit all fifteen Welsh mountains over three thousand feet: the Snowdon massif itself, the Glyderau, and the Carneddau in the north, with the lower Moelwynion, Moel Hebog range, Rhinogydd, Cadair Idris, Aran Fawddwy, and Dyfi hills extending south. More than 26,000 people live inside the park boundary; in the 2011 census, 58.6% of them spoke Welsh.

The geology underneath the park is what makes the surface so dramatic. Cambrian and Ordovician sedimentary rocks were folded and faulted during the Caledonian Orogeny around 400 million years ago, then intruded by Ordovician and Silurian igneous rocks. The slates that built the local economy come from low-grade metamorphism of Cambrian and Ordovician mudstones. The visible landscape, though, is a product of ice. Successive glaciations carved the cwms, the hanging valleys, the rounded ridges, the U-shaped troughs that now hold lakes like Llyn Padarn and Llyn Peris at Llanberis. The last ice age ended just over 11,500 years ago. In 1841 Charles Darwin visited Cwm Idwal and recognised the marks of glaciation in its rocks - one of the moments that helped found glacial geology as a science.

Snowdonia is one of the wettest parts of the UK. Crib Goch, the famous knife-edge ridge above Pen-y-Pass, holds the British record: an average of 4,473 millimetres of rain a year over the thirty years measured before the mid-2000s. That is more than fourteen feet of water falling on a single hillside. The mountains catch westerly weather coming off the Irish Sea and squeeze it dry. The result is the lichen, the moss, the dripping cliffs of native oak woodland, and the swift, clear rivers that drain to Cardigan Bay. It also gave Snowdonia its slate. The same wet climate that made the rock easy to split and the workings hellish to work in built a quarrying industry that, at its 19th-century peak, employed 12,000 men.

Llywelyn the Great in the 13th century used the title Tywysog Cymru ac Arglwydd Eryri - Prince of Wales and Lord of Snowdonia. His grandson Llywelyn ap Gruffudd kept the title until his death in 1282. When Edward I conquered Wales, he ringed Eryri with stone castles at Caernarfon, Conwy and Harlech, now collectively a UNESCO World Heritage Site. Earlier the Romans had reached this far by AD 77 or 78, leaving forts at Segontium (Caernarfon) and an amphitheatre at Tomen y Mur. Earlier still, Bronze Age farmers laid out the irregular field systems and burial chambers - like Bryn Cader Faner - still visible on the moors. Pilgrims crossed these mountains in the medieval period, traveling between Bangor, Holyhead and Bardsey Island. The high ground of Eryri has been worked, fought over, prayed in, and crossed for at least five thousand years.

Northern Eryri is the only place in Britain where the Snowdon lily (Gagea serotina), an arctic-alpine survivor of the post-glacial flora, grows. The Snowdonia hawkweed (Hieracium snowdoniense) grows only here, nowhere else on Earth. The rainbow-coloured Snowdon beetle (Chrysolina cerealis) shares the same uniqueness. Around them, on a larger scale, otters, polecats and pine martens move quietly through the valleys. Red kites soar above the southern hills. Ospreys nest where they can. Feral goats - descended from herds brought by Neolithic farmers around 5,000 years ago - graze on terrain too steep for sheep. Nearly twenty percent of the park's total area is protected under UK and European designation. The mountains carry their own ecosystem, finely tuned to the latitude, the altitude and the weather, and the people who manage Eryri are working to keep that intact even as 3.89 million visitors arrive each year.