The Poás Volcano, (), is an active stratovolcano in central Costa Rica and is located within Poas Volcano National Park. It has erupted 40 times since 1828, including April 2017 when visitors and residents were evacuated. The volcano and surrounding park were closed for nearly 17 months, with a 2.5 kilometer safety perimeter established around the erupting crater. As of September 1, 2018 the park has reopened with limited access only to the crater observation area and requires a reservation to be made on the National Park Website. Adjacent trails to Lake Botos as well as the museum at the visitor center remained closed. The volcano erupted briefly twice in September 2019. Crater lakes There are two crater lakes near the summit. The northern lake is known as the Laguna Caliente ("hot lagoon") and is located at a height of 2,300 m in a crater approximately 0.3 km wide and 30 m deep. It is one of the world's most acidic lakes. The acidity varies after rain and changes in volcanic activity, sometimes reaching a pH of almost 0. The bottom of this lake is covered with a layer of liquid sulfur. Acidity, lake temperature fluctuations, and a complex, toxic sulfur and iron chemistry (conjectured to be like conditions on early Earth and Mars) limits aquatic life to specially-adapted Acidiphilium bacteria. Above ground, acid gases create acid rain and acid fog, causing damage to surrounding ecosystems and often irritation of eyes and lungs. Lake Botos, the southern lake, fills an inactive crater, which last erupted in 7500 BC. It is cold and clear, and is surrounded by a cloud forest within the National Park boundaries. Eruptive history On May 17, 1953, an eruption occurred that started a cycle that lasted until 1956. At least two people were reported missing. Poás was near the epicenter of a 6.1-magnitude earthquake in January 2009 that killed at least forty people and affected Fraijanes, Vara Blanca, Cinchona (the most affected area), the capital San José, and the Central Valley region of Costa Rica. There was also eruptive activity in 2009 involving minor phreatic eruptions and landslides within the northern active crater. Poás eruptions often include geyser-like ejections of crater-lake water. On February 25, 2014, a webcam from the Volcanological and Seismological Observatory of Costa Rica (OVSICORI) captured the moment a dark cloud exploded about 1,000 feet in the air from a massive crater of