It started as a distant roar somewhere in the Congolese jungle ahead of us.
We set off, in single file, along a narrow path, which grew more treacherous during the course of a furious afternoon rainstorm.
After two hours, the roar began to sound like waves crashing on a shingle beach.
"Please don't talk loudly when we're walking," said one of the armed guards. "This is for your safety."
We were in Virunga National Park in the eastern Democratic Republic of Congo - home to a spectacular collection of volcanoes and mountain gorillas, and in recent years to an alarming assortment of militias, foreign rebel groups, army factions and marauding gangs - the bitter-ender remnants of DR Congo's long wars.
Hence the odd combination of a dozen enthusiastic tourists and four armed park wardens on our jungle trek.
An hour later, we climbed a ridge and there it was - a vast, squat, black mound belching and spurting red lava several hundred metres into the sky.
As the lava turned to fall, it hardened into black hail which crashed onto the sides of a brand new mountain that was rising from the surrounding plain by several metres a day.
We all stood, transfixed, watching the eruption less than 400m away.
To the north, a solid grey river of lava vanished into the jungle and the distant plains.
When the wind changed during the night, hard flakes of ash fell like rain onto our tents.
Schools and clinics
The park authorities - who already take a growing number of visitors to see the gorillas in the area - have been quick to entice more tourists to see the new volcano that is now two weeks old, and likely to start slowing down very soon.
The atrocious roads and the poor security mean that Virunga remains something of an acquired taste - most tourists are either adventurous backpackers tempted briefly over the border from Rwanda, or the very wealthy who come in by helicopter or private plane.
The security threat in Virunga is real and constant - with 17 soldiers killed in the park this year and more than a 140 in the past decade.
But there have been no incidents involving tourists, who are only taken to the southern tip of the park, close to the town of Goma.
As the numbers grow - the park is hoping for almost 4,000 visitors this year - it is tempting to see the steady rise in tourism as another sign of increasing stability in DR Congo, where the economy is likely to grow by 6.5% this year.
It is certainly encouraging. The park's revenues are shared with local communities who are provided with schools, clinics and advice on how to protect the forest itself.
But officials are wary of reading too much into the mini-boom in tourism, noting that to the outside world eastern DR Congo remains "a war zone" and that although there are some signs of positive change, the country is a long way from emulating neighbouring Rwanda's dramatic transformation.
When our group emerged from the jungle, a crowd of local village men were waiting to offer their services as porters - for about $20 (£13) for a round trip - to the next batch of tourists.