At Nant-y-Garth Pass, the A525 Wrexham to Ruthin Road is susceptible to undermining due to the scouring action of an adjacent fast-flowing stream. Concealed scour cavities were suspected following manual probing but the full threat posed to the integrity of the carriageway and to the safety of road users remained uncertain. Ground penetrating radar was used along a 350 m stretch of the highway to investigate the number and extent of scour holes beneath the carriageway. The survey identified extensive subsurface voiding with distinct anomalies associated with shallow cavities measuring up to 9 m long by 0.2 m high and extending as far as the centre of the road. More subdued radar features were interpreted as scoured pockets where fine-grained debris had been washed out and the remaining material had been disturbed and loosened. The results provided a basis for prioritizing countermeasures designed to repair the highway foundations and ensure the public safety.