Volcanologists take the pulse of DR Congo's temperamental volcano

The trek up Mount Nyiragongo takes you through fog, wind and ash to a crater -- the brooding mouth from which Africa's most active volcano last month spewed rivers of lava and forced the evacuation of a city.

Four experts from DR Congo's Goma Volcano Observatory (GVO) took the trail last week to get a closer look at Nyiragongo, which briefly flared into life on May 22.

The five-hour ascent on Friday, along a path previously used by tourists, took the team to an altitude of 3,470 metres (11,384), transiting from tropical warmth and lush vegetation to grey, cold stoney slopes where nothing grows.

The visitors gained a dizzying view of the crater, which emits a kind of low-frequency growl, punctuated by explosions. At night residues of glowing lava can be seen at the bottom of its crater. 

Equipped with drones and camera systems, the team wanted to measure landslips and monitor the volcano for a potential collapse of the crater, said Christopher Horsley, a technical assistant at the OVG said.

"We are seeking cracks that opened during the eruption," said Bonheur Rugain, a specialist. 

On social media, rumours have spread that Nyiragongo is now dormant -- an idea that the scientists are eager to dispel. 

Nyiragongo, a strato-volcano nearly 3,500 metres high, straddles the East African Rift tectonic divide. Just 14 kilometres (nine miles) away lies its sister Nyamuragira, whose red glow also shows that it is dangerously active.

- Buried homes -

The team, which also included seven porters and two cooks as well as armed guards, spent the night in small visitors' huts on the edge of the crater. Ash covered the mattresses. The temperature dropped to as low as 10 degrees Celsius (50 degrees Fahrenheit) and the wind was strong.

Nyiragongo's eruption on May 22-23 spewed out lava that buried homes in its wake, stopping just short of the northern outskirts of Goma, a city of some 600,000 people.

In the following days, mighty tremors shook the city, and scientists feared a rare but potentially catastrophic event -- a "limnic eruption" under nearby Lake Kivu that would send carbon dioxide gas, dissolved in the depths of the water, up to the surface and suffocate everyone in the vicinity.

The authorities ordered the evacuation of 400,000 people as a precaution. The residents have now largely returned after seismic activity fell back.

Thirty-two people died from lava burns or asphyxiation, and two more died in accidents during the exodus.

In the previous major eruption in 2002, around 100 people died and swathes of eastern Goma were destroyed.

Nyiragongo's deadliest eruption, in 1977, claimed more than 600 lives.

Friday's overnight stay marked the second monitoring expedition to the volcano since the May 22-23 event. Guards from the nearby Vigunga National Park provided protection, in an area notorious for armed groups.

"Before the eruption, it was tough because we didn't have enough money" to organise the trips, said Celestin Kasereka Mahinda, the OVG's scientific director.

"We used to go irregularly, about once a month."