Only 12 percent of the world's active volcanoes are topped with lakes. Scientists study these lakes' chemistry to monitor and learn more about volcanic activity. Whereas the lakes are rare and beautiful, they can also be dangerous. Those atop active vents can bubble and overflow during an eruption, spilling boiling water down the volcano's slopes.
Near the summit of Mount Tongariro in New Zealand's volcanic region, the Emerald Lakes take their color partly from dissolved minerals.
About 6,850 years ago, Mount Mazama blew its top in a massive explosion that rained ash, dust and lava. Following the explosion, Mazama's top collapsed to form the caldera you see in this shot, taken in 2006 from the International Space Station. The caldera later filled with water and became what is now one of the world's largest freshwater lakes with the deepest average depth of any in North America.
One of the world's most active volcanoes, Kilauea harbors a summit caldera that is covered in fresh lava flows in this satellite image.
Look closely to spot two lakes in this colorized satellite image. The first, a crater lake, appears as a tiny blue area at the summit of the Santa Ana volcano, the highest point in El Salvador. The second, just behind and to the right of Santa Ana, is a much larger caldera lake inside the Coatepeque Caldera. The red coloring indicates vegetation on the slopes of the volcanoes.
The pale blue dot in the center of this image marks Crater Lake atop New Zealand's Mount Ruapehu, one of the most thoroughly studied and monitored volcanic lakes in the world. "Eruptions through the lake occur relatively frequently, changing the physical dimensions of the lake and posing a constant threat to human activities in the area," scientists at the University of California, Davis, wrote.