Aerial reconnaissance in 1989 revealed a semicircular structure lying just off the coast at Clynnog Fawr in Caernarfon Bay, North Wales. The similarity in shape of the structure to other coastal fish-traps in the area promoted further study. Several avenues of research were followed. Historical sources were scrutinized in an attempt to find a date for the construction of the structure. A hydrographic survey was used to obtain the exact position and shape of the anomaly. A survey of the cliff and beach directly inland of the site was also made.

The anomaly was seen to lie further out to sea today than would have been the case at the time of its construction, indicating an area of coastal erosion. In an attempt to understand this process, the marine and atmospheric forces acting on the area were examined. In association with the processes of coastal erosion, soil tests were undertaken to assess the nature of the material being eroded.

Using both historical and physiological evidence it was concluded that the anomaly was indeed a fish-trap, constructed at about the beginning of the 13th century. The results from the different avenues of study have shown a sea-level rise of about 2.84 m and a cliff recession of about 118 m since its construction.