The Krakatoa eruption of 1883 was one of the most violent events in modern history. The volcano, located in Indonesia, tore itself apart in a series of catastrophic explosions that affected the entire planet, including cooling average temperatures by over a degree.
Over the past century, the volcano has been building itself back up, as you can see in this picture
taken by the Earth Observing-1 satellite. It's over 2 km (1.2 miles) across now. It may be decades or centuries before another catastrophic eruption occurs, and in the meantime NASA keeps an eye on this sleeping giant. Image credit: Jesse Allen/NASA EO-1 team