The Spanish Cave — Geoffrey Household

A twelve year-old boy finds terror and adventure inside a Spanish cave

When his parents died, Dick stepped aboard a ship on the Thames and left England behind forever. He made his way to Spain, where he found work as a fisherman. Life at sea is difficult, but since learning to speak their language, Dick has found the Spanish to be the most hospitable people on earth. They call him Ricardito and treat him as one of their own. It is an idyllic life -- until the day he makes his discovery.
 
Dick is fishing with a friend when their net drags the skull up from the bottom of the forbidding Cave of the Angels. Although the locals warn against exploring the mysterious cave, Dick cannot resist. Dared by one of his friends to spend a night there, Dick ventures inside. Something terrible lurks inside the Cave of the Angels, but there is no creature of the sea that can frighten Ricardito.