Cors y Llyn ('bog of the lake') is small in area for a national nature reserve, only about one square mile, but nationally important for being a pristine example of an acidic bog and fen, filled with a great abundance of bog plants, and made scenically interesting by a scattering of stunted Scots pines, creating an enchanted landscape reminiscent of the high latitude wetlands of Scandinavia.
The bog is entirely ringed by a thin belt of trees, mainly birch, so is isolated from the surrounding farmland, adding to the sense of a foreign landscape. The reserve also contains a couple of ponds and a wildflower meadow, supposedly one of the best in Wales, supporting over a hundred species.
The bog would be completely inaccessible due to the watery ground, some parts of which have deep pools with floating vegetation, were it not for a half mile boardwalk trail that loops across, also coming close to the ponds and the meadow. The route is wheelchair-accessible, and a leisurely walk takes around half an hour.
The bog is named on some maps as just Llyn, or lake, since it was in older times an area of open water, but has gradually been filled with sediment and colonised by plants. The waters occupy two shallow basins, carved by glaciers during the last Ice Age, on a bench on the east side of a low hill; further east, beyond a moraine ridge, the land slopes down further to a small stream, Nant-y-Prophwyd, which is partially fed by seepage from the bog, northwards. The River Wye flows past on the west side of the hill, 3 miles north of Builth Wells.
Cors y Llyn National Nature Reserve is not signposted, and a little difficult to locate despite being just a third of a mile from the busy A470. Access is via a lesser road to the west, signed Cwm-bach Llechrhyd; along for 700 feet and left again on a road to the south that passes right through the grounds of Graig Goch Garage then reaches a T-junction in front of the reserve parking area. From here a path runs past the west edge of the wildflower meadow to the brink of the two ponds, large and small, both surrounded by tall reeds and boggy ground, then veers west into the birch woodland and on to the start of the loop section, for which clockwise (left) is the recommended direction. The path, now a wooden boardwalk, soon exits the trees and traverses the northern of the two basins, mostly treeless apart from a few dead trunks, and containing all of the varied riparian plants of the reserve. After 500 feet the path crosses through a narrow band of slightly higher ground into the southern basin, where the ground is a little drier, and here are found the stunted Scots pines, the most photogenic part of the reserve. The boardwalk then moves west, over a patch of more open terrain and re-enters the woodland, back to the junction.