in the formation of the present caldera. The lake now fills a large part of the caldera, reaching depths of up to . The caldera-forming eruption is known as Los Chocoyos eruption and ejected up to of tephra. The enormous eruption dispersed ash over an area of some : it has been detected from Florida to Ecuador, and can be used as a stratigraphic marker in both the Pacific and Atlantic oceans (known as Y-8 ash in marine deposits). A chocoyo is a type of bird which is often found nesting in the relatively soft ash layer. Since the end of Los Chocoyos, continuing volcanic activity has built three volcanoes in the caldera. Volcán Atitlán lies on the southern rim of the caldera, while Volcán San Pedro and Volcán Tolimán lie within the caldera. San Pedro is the oldest of the three and seems to have stopped erupting about 40,000 years ago. Tolimán began growing after San Pedro stopped erupting and probably remains active, although it has not erupted in historic times. Atitlán has developed almost entirely in the last 10,000 years and remains active, its most recent eruption having occurred in 1853. On February 4, 1976, a very large earthquake (magnitude 7.5) struck Guatemala, killing more than 26,000 people. The earthquake fractured the lake bed and caused subsurface drainage from the lake, allowing the water level to drop within one month. Ecological history In 1955, the area around Lake Atitlán became a national park. The lake was mostly unknown to the rest of the world, and Guatemala was seeking ways to increase tourism and boost the local economy. It was suggested by Pan American World Airways that stocking the lake with a fish prized by anglers would be a way to do just that. As a result, an exotic non-native species, the black bass, was introduced into the lake in 1958. The bass quickly took to its new home and caused a radical change in the species composition of the lake. The predatory bass caused the elimination of more than two-thirds of the native fish species in the lake and contributed to the extinction of the Atitlan grebe, a rare bird that lived only in the vicinity of Lake Atitlán. A unique aspect of the climate is what is referred to as Xocomil (of the Kaqchickel language meaning "the wind that carried away sin"). This wind is common late