The region first saw volcanic activity about 11 million years ago, and since then has seen four separate episodes of volcanic growth and caldera collapse, the most recent of which began about 1.8 million years ago and culminated in the formation of the present caldera. The lake now fills a large part of the caldera, reaching depths of up to 600 metres.
The caldera-forming eruption is known as Los Chocoyos eruption, and ejected up to 300 km(72 cu mi) of tephra. The enormous eruption dispersed ash over an area of some 6 million km²: it has been detected from Florida to Ecuador, and can be used as astratigraphic 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 volcanism 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 grown almost entirely in the last 10,000 years, and remains active, with its most recent eruption having occurred in 1853.
On February 4, 1976, a massive earthquake (magnitude 7.5) struck Guatemala killing more than 26,000 people. The earthquake fractured the lake bed causing subsurface drainage from the lake, allowing the water level to drop two meters within one month.
In 1955, the area around Lago de 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. So, a
