One of the most fascinating aspects of Guanacaste's volcanoes is their visible, geothermal activity. The Rincón de la Vieja National Park is famous for its "pailas," or boiling mud pots, which bubble and steam within the forest, alongside sulfurous fumaroles and hot springs. This is a place where you can directly witness the immense heat of the Earth. Further south, Tenorio Volcano provides a different kind of magic. While the volcano itself is dormant, its mineral-rich outflows are responsible for one of Costa Rica's most stunning sights: the Río Celeste. This river's startling, turquoise-blue color isn't a trick; it's a natural chemical reaction that occurs when minerals from the volcano mix in the water, creating a breathtaking natural wonder.