Most people come to this reserve to see the volcano Cerro Negro (726m and growing), one of the youngest volcanoes in the world. Almost every guide in León offers a guided hike to the top of El Hoyo, a shadeless, two- to three-hour climb into the eye-watering fumes of the yellow-streaked crater, from where you'll shimmy downhill on a volcano board. Afterward, relax in the deliciously cool Laguna de Asososca, a jungle-wrapped crater lake (a popular add-on).
Cerro Negro first erupted from a quiet cornfield in 1850, and its pitch-black, loose-gravel cone has been growing in spurts ever since. Other peaks worth climbing include the dormant Volcán Pilas (1001m), which last had gas in 1954; and El Hoyo (1088m), the park’s second-most active peak, which is basically a collapsed crater with fumaroles.