Gruta de las Maravillas Nature · Aracena, Sierra de Huelva, Huelva Karst cave more than 2,100 metres long, of which around 1,200 are open to visitors, with formations of stalactites, stalagmites and aragonite; the province's only major show cave, with more than 140,000 visitors a year. Photo: Junta Informa (CC BY-SA 2.0) What to see and do in Aracena