The silent build-up to a super-eruption. It is estimated that about 5-10 volcanoes worldwide are capable of producing a super-eruption that could catastrophically affect global climate. One of these volcanoes hides below the waters of Lake Toba in Sumatra and has caused two super-eruptions in the last one million years. But when will the next one be? Will there be any warning signs? To answer these questions, an international team of geologists developed an analysis of the levels of uranium and lead in zircons -- a mineral typically found in explosive volcanic eruptions -- to determine how long it took the volcano to prepare for its super-eruptions. Source 3b.