The Secret Lagoon spa in Iceland is a hidden gem, offering visitors a unique and natural hot spring experience. Established in 1891, it is the oldest swimming pool in Iceland, with water sourced from geothermal hot springs like Vaðmálahver and Litli Geysir. The warm, sulfur-rich water stays at a comfortable 38-40 degrees Celsius year-round, providing a relaxing and rejuvenating soak.