Poas: A Deep Dive into Costa Rica's Crater-Lake Volcano
Poas, rising in the highlands northwest of Costa Rica's capital, is one of the country's most visited and most dramatic volcanoes. Its enormous summit crater holds a milky, turquoise lake so acidic it can dissolve metal, and its periodic eruptions of steam, gas, and ash make it one of the most active volcanoes in Costa Rica. Accessible by road within a national park, Poas offers a rare close-up view of a restless and chemically extreme volcanic system.
A vast and accessible crater
Poas reaches about 2,697 metres and is crowned by one of the largest active craters in the world, more than a kilometre across. A road and trails within Poas Volcano National Park bring visitors close to the crater rim, where they can gaze across the steaming pit toward the acid lake below. This accessibility has made Poas one of the most popular natural attractions in Costa Rica.
The acid crater lake
The crater of Poas holds Laguna Caliente, one of the most acidic lakes on Earth. Fed by volcanic gases and hot fluids, its waters are so corrosive that they can strip metal and devastate surrounding vegetation through acid rain. The lake's colour and level change with the volcano's activity, and at times it has dried out entirely as heat and gas emissions intensify, only to reform in quieter periods.
A history of phreatic eruptions
Poas is known for frequent phreatic and phreatomagmatic eruptions, in which water in the crater system flashes to steam and drives explosions. These can hurl water, mud, and rock from the crater, sometimes in spectacular geyser-like bursts. Such eruptions, while often relatively small, can be dangerous to anyone near the rim and are a defining feature of the volcano's behaviour.
The 2017 eruption and closure
In 2017, Poas entered a phase of significantly increased activity, producing powerful phreatic eruptions that ejected material from the crater and prompted the closure of the national park for safety. The event damaged infrastructure near the crater and underscored that, despite its accessibility, Poas is a genuinely active and hazardous volcano. The park later reopened with enhanced safety measures.
Managing access at an active volcano
The reopening of Poas illustrates the challenge of balancing public access with safety at an active volcano. Authorities limit visitor numbers and time at the crater rim, monitor gas levels, and maintain shelters and evacuation procedures. This careful management allows people to experience the volcano up close while reducing the risk should activity escalate suddenly.
A cloud forest setting
Beyond the stark crater, Poas Volcano National Park protects lush cloud forest that thrives in the cool, wet highland climate. The forest is home to abundant birdlife and distinctive vegetation, and a second, extinct crater within the park holds the serene Botos Lake, a striking contrast to the acidic Laguna Caliente. The combination of active volcanism and rich ecosystems makes the park exceptional.
Monitoring Poas
Costa Rica's volcanological and seismological institute monitors Poas closely, tracking its seismicity, gas emissions, and the behaviour of its crater lake. Because the volcano can shift quickly from quiet degassing to explosive activity, continuous monitoring is essential both for protecting visitors and for understanding the chemistry and dynamics of this extreme volcanic system.
Explore on the map
Poas stands among Costa Rica's chain of volcanoes, alongside Arenal, Irazu, and Turrialba. Explore it on the interactive map — filter by country to see Poas among Costa Rica's volcanoes and to place this crater-lake volcano within the volcanic arc of Central America.