The Toba caldera, Sumatra, Indonesia

Lake Toba, on the Indonesian island of Sumatra is the site of the largest volcanic eruption in the past two million years.  The eruption occurred approximately 75,000 years ago, and released an estimated 2800 cubic kilometres of magma in the form of ash and pyroclastic flows.  Compare this with the catastrophic Tambora eruption of 1815: ~100 km³, the huge 1991 eruption of Pinatubo:  <10 km³ or the famous 1980 eruption of Mt. St. Helens: <1 km³. 

 

The 1991 Pinatubo eruption resulted in an average global temp drop of around 0.5 C that lasted for a couple of years.  The Tambora eruption and some other large historic eruptions had much more significant effects and are known to have created famine conditions that killed many thousands, if not millions of people around the world.

 

Some researchers believe that the Toba eruption might have caused the near extinction of humankind, dropping the numbers of our ancestors to less than 20,000, and eliminating any that were living in temperate regions of Europe or Asia.

For more on Toba see: http://hvo.wr.usgs.gov/volcanowatch/2005/05_04_28.html