The Poás Volcano is an active stratovolcano in central Costa Rica. Poás has erupted 39 times since 1828. There are two crater lakes near its summit. The northern lake is known as the Laguna Caliente and is located at a height of 2,300 m in an approximately 1.7 km wide and 290 – 300 m deep crater. It is one of the world’s most acidic lakes. Acidity of the lake varies after rain and changes in volcanic activity, sometimes reaching a pH of almost 0; consequently, it supports little or no aquatic life. The bottom of this lake is covered with a layer of liquid sulphur. Acid gases create a phenomenon of 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 7500BC. It is cold and clear, and is surrounded by a cloud forest located within the Poás Volcano National Park. 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. It also has had 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.