Cave of el Agua y sumidero del Hurón Nature · Basconcillos del Tozo, Páramos, Burgos Image of Basconcillos del Tozo, not of the place itself. The River Hurón sinks underground at the Cueva del Agua and runs some four kilometres of underground galleries before coming back to the light as the River Rudrón, one of the greatest karst features of the Geopark. Photo: Zarateman (CC0) What to see and do in Basconcillos del Tozo