El Soplao (Cantabrian: El Sopláu) is a cave located in the municipalities of Rionansa, Valdáliga and Herrerías in Cantabria, Spain. It is considered unique for the quality and quantity of geological formations (speleothems) in its 17 miles length, 6 of which are open to the public. In it are formations such as helictites (eccentric stalactites defying gravity) and curtains (draperies, or sheets of calcite, sometimes translucent, hanging from the ceiling). Its formation dates back to the Mesozoic, in particular the Cretaceous period 240 million years ago. The entrance is at 540 metres in the Sierra Soplao Arnero.

The cave was accidentally discovered during drilling for mining, subsequently being exploited for the extraction of minerals. During its operation, many local families were supported by the income they got from the mining, combined with farming livestock. After decades of neglect, speleology, and in particular the Cantabria University Speleology Club since 1975, have discovered its true geological value.

On July 1, 2005 the Government of Cantabria opened it to the public and publicised it internationally, after development for tourism and protection rules prohibiting any further activity by the discovering club. Research goes on, resulting in the study of amber deposits, the recognition of underground stromatolites formed by manganese-oxidising bacteria, and a new mineral form, zaccagnaite-3R.

The cave recently started holding music concerts. Artists like Bertín Osborne or Nando Agüeros have played inside the cave for hundreds of people in 2016.

El Soplao is just one of at least 6,500 caves in Cantabria.