The poisonous plume has withered forests and crops, but also contains massive amounts of fluorine, which has seeped into water supplies. Two years ago the Kiza family were standing outside their home on the edge of Goma, watching the spectacular fireworks of the volcano in the distance. Suddenly the ground split open beneath their feet.”I was sitting on the crest,” said Furaha Kiza, 13, pointing to the spot where her family once lived. “Then the ground started shaking and fire came out.” The family fled, but Furaha’s father, Evariste, who was too sick to run, was consumed by the tide of lava.Next time they should have more warning.

A series of digital seismographs around the volcano provide a continuous stream of data to the observatory in Goma. Physical and chemical warning signs should allow volcanologists to predict the next eruption three weeks in advance, said Mr Durieux.For now, there are more immediate worries. “I prefer not to think about it,” said Mr Durieux,Lava flows from Nyiragongo – one of two very active volcanoes along the Virunga Mountains – are fast, up to 25mph, and its centre of eruption is unpredictable. “We know it will erupt again, we know how and we know where,” he said, standing on a crumbling mound of lava rock on the edge of town. “There would be no possibility of escape,” said Jacques Durieux, a French volcanologist of the Goma Volcano Observatory, an international project that monitors Nyiragongo’s changing moods.The centre of eruption has shifted towards the town since the volcano went quiet. “The only thing we don’t know is when.”Another, even more catastrophic, scenario is possible. If the centre of eruption shifted under nearby Lake Kivu, it could trigger a cloud of deadly gases, killing more than four million people.

In 1986 a lethal emission from Lake Nyos in Cameroon claimed 1,800 victims. Next time – anywhere between three weeks and 30 years from now – lava will gush up through the rutted streets, giving half a million people no time to flee. And the next time it could explode under Goma itself, taking tens of thousands of lives. Neighbourhoods burned, shops were swamped, petrol stations exploded.

The burning river crawled across the airport runway and gutted the main cathedral. Remarkably, only about 100 people died.Today the danger is ever-present. The volcano is calm for now, but could blow at any time, scientists warn. Fountains of lava burst from its flanks and snaked across the plain to Goma, the provincial capital of 500,000 inhabitants, 10 miles to the south It was an awesome tour de force. “The volcano must be angry,” he said.Two years ago yesterday Mount Nyiragongo, a volcano near the Democratic Republic of Congo’s eastern border with Rwanda, erupted spectacularly.