Pen Park Hole is currently the UK's only known hydrothermal cave system. Situated under a large housing estate in north Bristol, it contains one of Britain's largest natural underground cavities, the roof of which is only a few metres below the surface. Discovered in 1669, the cave has had a long history of investigation, including being the object of the world's first published cave survey in 1683. In 2006, a telecommunication company submitted a planning proposal to erect a transmitter close to the site. Given the sensitive nature of the cave and potential uncertainties in the accuracy of a more recent cave survey, Bristol City Council planners required a better fix on the location of the cave. The urban surroundings prevented traditional geophysical techniques being used, and a ground penetrating radar survey failed because of thick residual clay soils over the site. Instead, radio-location was employed to check on the accuracy of the existing cave survey, allowing it to be geo-rectified in relation to the surface. The confirmed reliability of the survey, coupled with an assessment of the cave geomorphology, has allowed the planners to assess the potential for the proposed mast works to intersect the cave or any associated undiscovered voids.