Do not make St. Patrick Angry!
The cache is on the old Tennant Canal that runs from St Thomas in Swansea to Neath. It is surrounded by wetlands - note that in winter inparticular the path from Jersey Marine can be totally waterlogged, but it was fine today (May 2024). Doable on a hybrid bike, but note that the path is unsealed in places.
According to the folklorist Wirt Sykes, Crumyln Lake is a haunt of the Gwragedd Annwn - which is variously translated as "Wives of the Lower World", "Dames of Elfin Land" & similar. Beneath the dark waters of this lowland fen is a full town of the Gwragedd Annwn. The fairy bells of their towers can sometimes be heard to ring and, at least in 1880, people claimed to have glimpsed the battlements of beautiful castles beneath the water. The Gwragedd Annwn are unique British fairies that are found only in the isolated lakes and rivers of Wales, but never in the sea. These fairies can ascend into the upper world and are sometimes seen as women clad all in green.
Their origin is suprising - do not make St. Patrick Angry:
"The way the elfin dames first came to dwell there was this: A long, ay, a very long time ago St. Patrick came over from Ireland to visit St. David of Wales, just to say 'Sut yr y'ch chwi?' (How d'ye do?); and as they were strolling by this lake conversing on religious topics in a friendly manner, some Welsh people who had ascertained that it was St. Patrick, and being angry at him for leaving Cambria for Erin, began to abuse him in the Welsh language, his native tongue. Of course such an insult could not go unpunished, and St. Patrick caused his vilifiers to be transformed into fishes; but some of them being females, were converted into fairies instead."
Wirt Sykes, British Goblins: Welsh Folk-lore, Fairy Mythology, Legends and Traditions, 1880
Nowadays Crymlyn Bog is the largest lowland fen in Wales & an important wetland habitat. The reed and sedge beds are home to a wide variety of plants, birds and insects such as the fen raft spider - Britain's largest and rarest spider.