Goma - Leaning over the rocky ledge, Jean-Bosco Simpeze peered into the heart of the volcano. Below, a cauldron of scarlet lava spat, bubbled and boiled. Giant clouds of acrid gas billowed out. The gurgle of churning magma filled the air.
Simpeze, a park ranger, turned away and faced the lush green plains below. Such massive gas emissions were not normal. "The volcano must be angry," he said.
Two years ago Mount Nyiragongo, a volcano near the Democratic Republic of Congo's eastern border with Rwanda, erupted spectacularly. Fountains of lava burst from its flanks and snaked across the plain to Goma, the provincial capital of 500 000 inhabitants, 16km to the south. It was an awesome sight.
The danger is ever present
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.
The danger is ever present. The volcano is calm for now, but could blow at any time, scientists warn. And the next time it could explode under Goma itself. "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. Next time - anywhere between three weeks and 30 years from now - lava could gush up through the rutted streets, giving half a million people no time to flee. "We know it will erupt again, we know how and we know where," Durieux said, standing on a crumbling mound of lava rock on the edge of town. "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. "I prefer not to think about it," Durieux said.
Lava flows from Nyiragongo - one of two very active volcanoes along the Virunga Mountains - are fast, and its centre of eruption unpredictable. 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, Durieux said.
For now, there are more immediate worries. Standing on the pitch-black soil outside the observatory, Professor Dario Tedesco of the University of Naples pointed to the plume of smoke churning from Nyiragongo's cone. Satellite images show it is emitting more than 54 000 tons of sulphur dioxide each day.
The poisonous plume has withered forests and crops, but also contains massive amounts of fluorine, which has seeped into water supplies. Levels in some places are above 10 times the recommended maximum, and villagers show signs of dental fluorosis, or rotting teeth. The volcanic activity is responsible for another eerie phenomenon. Hollows in the ground are filled with pools of carbon dioxide, which has seeped through the soil.
The cracks are known as mazuku - Kiswahili for "evil winds" - and their victims are usually children. Last month Regina Kabuo sent her daughter Baseme to buy sugar, but she never returned. Taking a short-cut through a mazuku, the five-year-old was overcome by the lethal gas. "We found her there," funny pictures said the grieving mother, pointing to a crevice. Geochemist Mathieu Yalire tied a burning rag to a long stick and dipped it into the hollow. About a metre above the ground the flame was extinguished.
Despite the swelling threat, Goma continues to grow. At current rates the population will reach 1 million by 2015, and aid workers worry about a potential cholera epidemic after an eruption. But the obvious step - to move the city - "is nearly impossible", Durieux said. "Never in the history of man has a city evacuated before a catastrophe. People always come back and rebuild in the same place."
In Congo, still reeling from five years of war, the idea of a relocation is even more fanciful. Mount Nyiragongo is one of many hazards, ranging from rapacious gunmen to HIV/Aids, that stalk Goma's weary inhabitants. Still, some are embracing the idea. Alphonse Kassole, a butcher whose business was destroyed in the last eruption, is leading a campaign to have Goma moved 40km along the lake, and has formed a lobby group called Alternative to Goma. "This town has no future," Kassole said. "When you build a house here, you can't be sure your son will be able to live in it. "But for now, we have nothing better."