The 12,016-hectare Parque Nacional Volcán Arenal (Arenal Volcano National Park, tel. 506/2461-8499, 8 a.m.–4 p.m. daily, last entrance at 3 p.m., $10) lies within the 204,000-hectare Arenal Conservation Area, a polyglot assemblage protecting 16 reserves in the region between the Guanacaste and Tilarán mountain ranges, and including Lake Arenal.

The park has two volcanoes: extinct Chato (1,140 meters), whose collapsed crater contains an emerald lagoon, and active Arenal (1,633 meters), a picture-perfect cone.

Arenal slumbered peacefully throughout the colonial era. On July 29, 1968, it was awakened from its long sleep by a fateful earthquake. The massive explosion that resulted wiped out the villages of Tabacón and Pueblo Nuevo. The blast was felt as far away as Boulder, Colorado.

Thereafter its lava flows and eruptions have been relatively constant, and on virtually any day you could see smoking cinder blocks tumbling down the steep slope from the horseshoe-shaped crater—or at night, watch a fiery cascade of lava spewing from the 140-meter-deep crater. Some days the volcano blows several times in an hour, spewing house-size rocks, sulfur dioxide and chloride gases, and red-hot lava.

The volcano’s active vent often shifts location; for the past decade it has been on the northern side, but in 2008 a collapse at the crater rim shifted the predominant lava flows to the southern side. Explosions and eruptions, however, occur on all sides.

The volcano entered a particularly active phase in 2005, with an average of 5–8 “big boom” explosions daily. In September 2010, however, it suddenly went quiet and at press time remained so. MINAE’s Comisión Nacional de Emergencias has set up four “safety zones” around the volcano and ostensibly regulates commercial development. It’s highly arbitrary, however, and any cataclysmic eruption would devastate the entire area.

The Arenal Observatory Lodge (tel. 506/2479-1070, www.arenalobservatorylodge.com) has a small but interesting Museum of Vulcanicity.