WPR has had limited use in freshwater archaeology and can provide superb 2D data over sites with appropriate water type and depth, such as exists in this case study at Castlewellan Lake, Northern Ireland. Here, a submerged crannog (Prehistoric, usually Bronze Age to Medieval human structure in water) is only shown on one historic map, yet is not presently visible nor on other historic/recent maps. This work used a desktop study, sonar and WPR to characterise the submerged structure as a crannog. The asymmetry, bathymetric position, upper surface (rock slabs) and general topography of the crannog are determined, yet the variable makeup of its interior and sides allow numerous theories and possibilities for further, probably non-invasive investigation. The limits, problems, data processing and uses of WPR for archaeology in this environment are described, some of which may be useful in other studies of freshwater archaeology, including crannogs, flooded dwellings, walkways/piers/jetties and military archaeology.