In the Canary Islands, groundwater is the main source of drinking water. Groundwater mines have been the system used by the engineers of the archipelago to collect water from the ground. The Canary Islands are volcanic with soils characterized by being rich in uranium, the disintegration of which gives rise to radon gas. In this study, radon gas levels in the mines on two islands of the archipelago have been measured to study exposure to this gas in the galleries. Results show values much higher than the European regulatory limit concentrations.