Cave of El Soplao

The site stretches across twenty kilometers of underground passages filled with stalactites, stalagmites, columns, and rare helictite formations that grow sideways or even upward. These mineral structures cover ceilings and walls in a dense network of shapes and textures.

Miners working in the early twentieth century uncovered the natural formation while searching for ore in the Sierra de Arnero. Their work revealed chambers that had remained hidden for thousands of years.

The name comes from mining language and describes air currents moving between tunnels and natural chambers. This link between industry and geology shaped how locals understood the underground terrain for decades.

The site sits 83 kilometers from Santander and maintains a year-round temperature around twelve degrees Celsius (54 Fahrenheit). Tours require advance booking, and visitors should bring warm clothing and shoes with grip.

Within the complex lies a significant amber deposit containing fossilized insects that offer researchers insight into ancient life forms. The inclusions trapped in resin date back 90 to 110 million years and rank among the oldest of their kind in Europe.