In a steady trickle teenage boys push their way down a dusty road to the bustling city of Goma, their bicycles buckling under the weight of 100-pound (45-kilogram) sacks of charcoal, or makala as it's known here.
The boys are part of an illegal trade that may pose the biggest threat to one of the most pristine places on the planet, the Democratic Republic of the Congo's Virunga National Park. (Read: Virunga National Park Sees Its Worst Violence in a Decade, Director Says.)
The park's dense forest is rapidly being depleted of its trees to satisfy the almost insatiable demand here for charcoal, which is used for cooking and heating by the millions of people living in this troubled region.
The lucrative charcoal trade is not only wreaking havoc on the park but also on its most famous inhabitants, the rare mountain gorillas.
Conservationists believe last month's execution of four mountain gorillas inside the park was carried out by people associated with the charcoal trade who want the park unprotected.
"The gorillas have become a hindrance for the charcoal trade," said Emmanuel de Merode, director of WildlifeDirect, a conservation group based in the Democratic Republic of Congo and Kenya that supports the park rangers working in Virunga.
"There's a very strong incentive for these people to kill the gorillas."
(Editor's note, August 17: Since this story was filed on August 16, rangers announced on their blog that one more gorilla has been found dead as a result of the July attack; her infant is still missing and presumed dead, bringing the total to six.)
Situated on the country's eastern border, with Rwanda and Uganda to the east, Virunga is Africa's oldest national park and boasts the highest biodiversity on the continent.
More than half of the world's 700 remaining mountain gorillas are found in Virunga.
But the park has been torn apart over the years by a procession of armed groups—from ragtag rebel militias to foreign armies—fighting over its natural riches.
"The last 15 years of Congo's history have been defined by the illegal exploitation of natural resources," de Merode said. "The charcoal trade definitely fits into that reality."
He estimates that the charcoal trade in Goma, a city of 500,000 people, alone is worth 30 million U.S. dollars.
"When you talk about charcoal, people think of this mom-and-pop, small-scale business, but it's not that at all," de Merode said. "It's a massive industry."
Much of the trade is connected to neighboring Rwanda, which has maintained a strong influence in eastern Congo ever since its troops drove out militiamen hiding here after the 1994 Rwandan genocide.
In contrast, Congo's central government, based in Kinshasa more than 1,000 miles (1,600 kilometers) west, has little political say over what goes on in the eastern part of its territory.
In 2004 Rwanda passed a law banning the production of charcoal within its borders.
That has put enormous pressure on the country to find charcoal for its nine million people somewhere else, de Merode said.
"Rwanda is unsustainable in terms of natural resources within its own borders, so it has to look to the outside," he said.
"What's happened is that there's only one real source of charcoal for Rwanda, and that's Virunga National Park."
The charcoal is mostly made inside the forest by small-scale producers, Virunga rangers say.
After a tree is cut down, the large branches are used to build a makeshift dome, which is covered with mud and set on fire. The mud makes the wood burn more strongly and form charcoal, a process that takes a couple of days.
The producers are organized into local associations, which de Merode and other sources working in the area claim are controlled by Congolese military officials. The officials exact a tax on both the charcoal's production and its transportation, the sources claim.
A strong military presence is clearly visible in and around Virunga National Park, with soldiers manning frequent roadblocks and mingling with villagers.
The soldiers have reportedly not received paychecks in years, and rangers say some may turn to the charcoal trade and other illegal activities to support themselves and their families.
"The military is put in the park because of the armed bandits that operate there, but they're not paid, so they start making charcoal instead," said Virunga ranger Paulin Ngobobo, in his office in Goma.
Ngobobo has been in charge of Virunga's southern sector, where the gorillas live, for just over a year.
"We'll get a report from a military commander saying we cannot patrol the park for a certain time because of military maneuvers, but what they're actually doing is cutting down trees and poaching," he said.
A military official in Kinshasa, who did not want to be identified, admitted that military personnel in eastern Congo operate in large part independently from the government.
Other political observers say they believe the involvement in the charcoal trade by military officials stationed in eastern Congo is probably done without Kinshasa's approval.
Ranger Beaten by Poachers
Confronting the people in the trade is a dangerous business, as Ngobobo has repeatedly learned.
Earlier this year, while lecturing villagers about the threats of the charcoal industry to Virunga, Ngobobo was arrested by military officials, stripped of his shirt, and flogged in front of the crowd, he said.
In addition to such alleged reprisals, Ngobobo also faces challenges in convincing local villagers to shun the charcoal business.
"Everyone is making money off this trade," he said.
"The population is very poor. It's impossible for them to see the value of the park. They see it as another obstacle."
Ngobobo also has to battle what he says are some corrupt officials within the park service, who are allegedly involved in the charcoal trade as well.
It's a problem Ngobobo refers to as "internal poaching."
"Most of the park officials risk their lives to protect the park, [but] there are some people in the park service who are in collaboration with the military and the poachers," he said.
Shortly after Ngobobo posted an article on WildlifeDirect's blog on the illegal charcoal trade, he was arrested and placed in the custody of a military tribunal in Goma for two days on charges of negligence.
According to court documents, Ngobobo has been accused of neglect in the death of a Chinese tourist who fell into a nearby volcano. He is also charged with furnishing false information about the charcoal trade and obstructing the investigation into the gorilla killings.
Ngobobo says the charges are politically motivated, brought against him by officials involved in the charcoal trade who want to see him removed.
In a telephone interview with National Geographic News, Ngobobo's former supervisor, Honore Mashagiru, dismissed those allegations.
"People say things, but where's the proof?" he said. "It's not true. It's not true."
Mashagiru said Ngobobo has become the target of the charcoal traders because "he has not communicated well with the community about the issue."
Ngobobo is still facing court charges and must report daily to the tribunal.
Meanwhile, Norbert Mushenzi, a park service director who has been in charge of the northern sector of Virunga, has been assigned to Ngobobo's post to protect the gorillas.
In recent days, Mushenzi, who also has a history of speaking out against the charcoal traders, and his rangers have detained about 50 women whom they caught making charcoal in the park.
"Act of Sabotage"
Both Ngobobo and de Merode are convinced that the execution of the gorillas last month is linked to the charcoal trade.
"None of the gorillas was cut up, and there was a baby still on one of the mothers," de Merode said.
"In the history of gorilla conservation, there's never been incidents like these where a group is attacked not for meat or baby gorillas."
A baby gorilla can fetch thousands of dollars on the illegal wildlife market, he added.
The mass execution was also identical to the killing of a female gorilla 8.7 miles (14 kilometers) away on June 8.
"In terms of the whole build-up over the last year, it's a very strong case for it being planned and being vindictive," de Merode said.
"We believe this was an act of sabotage by the people in the charcoal business who want to see the gorillas dead."
De Merode says the Rwandan authorities should seek to clamp down on the charcoal trade, which he believes would lead to greater protection for the mountain gorillas.
"I'm not saying the Rwandans are responsible for killing the gorillas, but they can help resolve the problem," he said.