Lake Toba is in the middle of Northern Sumatra, Indonesia. Known locally as a caldera, or the crater formed by a volcanic eruption, Lake Toba is big, measuring eighteen by sixty miles across and as deep as five thousand feet in places. The size of the lake is attributed to Toba's eruption, the single largest that has occurred anywhere on earth within the past two million years.

About 74,000 years ago a high volcanic mountain that stood on the area, now occupied by Lake Toba, erupted and blew skyward a mass of ash and volcanic debris that was three thousand times as big as the total amount that erupted from Mount St. Helens in 1980. The entire subcontinent of India was covered with ash. All around the globe sunshine was reduced and temperatures dropped by about 3 degrees, and stayed at that level for years. During that time, throughout the world, millions of all forms of life died. Thousands of species vanished.