Introduction to underground caves: The underground caves are one of the many mysteries of Easter Island. The famous Norwegian anthropologist and marine explorer Thor Heyerdahl first discovered the underground caves during his scientific expedition to Easter Island in 1955. secret. In the past, there were many secret caves on the island, which were the property of specific families, and only the most important members of the family knew the entrance to each secret cave. These secret caves were used to hide valuables or to hide in times of war.