Crymlyn Bog (Welsh: Cors Crymlyn) is a nature reserve and a designated Site of Special Scientific Interest of international significance, near Swansea, south Wales.
It is the largest area of lowland fen in Wales and lies immediately to the eastern side of Kilvey Hill just north the industrial area of Crymlyn Burrows.
Plantlife found in the bog is more typical of that found in East Anglia. Habitats range from swamps, carr (fen), water meadows and tall reed beds to waterlogged scrub consisting mainly of willow, where wetter areas merge with woodland. There are a number of plants found in the wetland which are rare in Great Britain. Examples of which include the slender cotton grass and lesser water plantain. In 2003 surveys of Crymlyn Bog, Pant-y-Sais Fen and the inter-connecting wetlands identified the area as one of only three locations in the UK at which the fen raft spider is found.[1] The extent of the population is unknown but the quality of habitat at the site is considered good enough for the population to be stable.[2]
The reserve is a heaven for birds. Predatory visitors like the hen harrier, buzzard, hobby and the occasional marsh harrier visit the site regularly. The site provides an important refuge for a range of wetland birds like the bittern, water rail, sedge and reed warblers, bearded tit and grey heron, which can often be seen or heard there.
There is a visitor centre in the bog which is frequented by nature and bird watching enthusiasts.
It developed in ancient times in the estuary of the River Clydach and River Neath which once flowed through the present day site of the bog.
Part of the land was previously occupied by a BP oil refinery,. Pany y Sais fen was largely owned by the CEGB who used the fen to dump PFA and by Swansea City Council who used is as a landfill. Small parts were owned by the Neath and Tennant Canal Company and small parts of the canal still exist at the site.
With the surrounding industrialisation, encroaching residential districts and a polluting waste dump beside it, it was uncertain that the bog would survive. Andrew Lees, who later became Campaigns Director of Friends of the Earth started a campaign with the Swansea branch of FoE to protect the bog. At the northern limb of the bog at Pant-y-sais fen, there is a memorial to Mr Lees, which carries a quotation from him: "At some point I had to stand up and be counted. Who speaks for the butterflies?"