"There is another wonder in the region of Cinlipiuc. There is in that place a spring called the Well-pool of Guur Helic. No stream flows out of it or into it. Men go to the spring to fish - some go to the eastern part of the spring and draw out fish from that part, some to the right, some to the left, and the west, and fish are drawn from those parts, and different types of fish are drawn from each part. It is a great wonder to find fish in the spring while there is no stream flowing in and none out of it, and in there they find four types of fish, and by no means is it deep! The depth is at every point to the knees; it is twenty foot in length and breadth; the bank is high that holds it from each direction."1,2,3,4,5
This Wonder is both a biological and watery one. The specific location is currently unknown, but the "country of Cinlipiuc" is likely to be the old kingdom of Cynllibiwg the region between the Severn and the Wye6,7. The steep banks around this pool and the constant depth suggests this is a constructed feature8, and there are two possible man-made features this might be: a purposefully-built fish-holding pool9, or a enclosed spring10. Although no one knows where this wonder is, we can suggest three contenders: Pwll-Helyg in Cardiff11,12 (Visit Details), the Well of Pen-Y-Fan13,14, and St. Noe's Well, Skenfirth15. Ultimately, however, a knee-deep pool is easily filled in 1200 years16.
You may like to read more about the Myth or Science of this wonder.