Cueva de El Soplao in Rionansa

You're standing at the entrance to El Soplao, a cave in the mountains of Cantabria, in northern Spain. Take a look around. You're at about 540 metres up, in the Sierra de Arnero, a ridge that runs parallel to the Cantabrian Sea. The cave system beneath your feet stretches for almost eighteen kilometres, on several levels. And it's just one of at least 6,500 caves in Cantabria.