A natural spring line can be found at the northern end of the reserve, from these springs a stream would have flowed. This stream was dammed in the 1950's to flood part of the valley creating a lake of some 4.5 acres in size in order to increase the fishing interest. The lake has subsequently overgrown with aquatic vegetation leaving open water of some 0.5 acre. The site was notified in 1987 as a Site of Special Scientific Interest primarily because of its high botanical interest. This reserve contains livestock so please keep dogs on a lead.
Pentrosfa's swampy areas are good places for Snipe and Jack snipe to roost in the winter. Other birdlife includes Bullfinch, Skylark, Linnet, Reed Bunting and Song Thrush. All three species of woodpecker have been observed in the wooded areas, and overhead Red Kites, Buzzards and Ravens patrol.
Interesting species associated with the water and aquatic vegetation include Water Rail, Sedge Warbler, Kingfisher, and specialist invertebrates such as the Water Ladybird Anisosticta 19-punctata, Water Scorpion Nepa cinerea, and a very scarce small black Rove Beetle Stenus oscillator which is particular to marshy places. During summer many species of insects emerge including Azure Damselfly Coenagrion puella, Common and Southern Hawker Dragonflies Aeshna juncea and A. cyanea along with wetland craneflies and hoverflies. The thistles on the dryer slopes provide a food source for Goldfinches, Ringlet and Meadow Brown Butterflies, and the grassland is also good hunting ground for Owls.
Mammals prove more difficult to view but Water Vole, Hare and Polecat have all been recorded here. Frogs and Smooth Newts can be found among the sedges, Northern Marsh-Orchids, Broad-leaved Cotton-sedge and Bog Violets growing on the damp soils. Pentrosfa Mire's rarest species is a lichen discovered on willow twigs which is new to science (a member of the Arthonia genus). Over 100 species of lichen have been found on this reserve.
From Llandrindod Wells lake, follow the road for the golf club. Go over the cattle grid and park in the lay-by on the right past the church (NGR: SO 064 601). Follow the public footpath through the churchyard, exit into a field. Turn right and leave field through hidden wooden gate on right. Descend steps and bear right on track, bear left almost immediatedly following public footpath signs around the farmhouse. Turn left on track and follow track for 400m until you see the reserve on your right (NGR: SO 062 598).