Booby traps, tunnels, mines and dense woodland cover thousands of miles.
The Nigerian military's push to invade the Sambisa Forest, the last stronghold of the Islamist militant group
Government forces have taken over numerous Boko Haram bases in the forest in Nigeria's northeast, rescued hundreds of women and children and released aerial images of terrorists retreating, but it has yet to capture the top leaders of the group or many of its fighters.
Until the military clears the vast forest of militants, Boko Haram's deadly raids on roads, villages and towns will continue, with more lives lost and no end to the paralysis of transportation and trade in the region. The group, fighting to establish an Islamist state in Nigeria, has killed and abducted thousands of people, although estimates of casualties vary widely.
Nigeria's military, criticized for much of last year for running away in the face of Boko Haram attacks, has recently retaken dozens of villages and towns seized by the group, including Gwoza, the headquarters of its self-declared caliphate. The militants have been driven back into camps in the forest, near the border with Cameroon. Analysts say there may still be several thousand militant fighters, but caution that accurate estimates are unavailable.
Nigerian troops pushed along a dirt road into the forest last month but were forced to retreat. A soldier and three members of an accompanying local civilian defense unit, probably acting as scouts, were killed when their vehicle hit a mine, according to Nigerian news media and unnamed military officials. The densely forested area off the road was also heavily mined.
Military spokesman Chris Olukolade said in a statement last week that the army continued to encounter land mines and booby traps. Some Boko Haram fighters have been killed, he said, although he did not give a number.
The forest is about 60 miles from the village of Chibok, where 276 schoolgirls were kidnapped last year. Rumors abound that some of the girls, who are believed to have been split up and married off to fighters or forced into slavery, may be prisoners in Boko Haram forest camps, but there's no evidence, analysts said. None of the Chibok girls have been among those freed so far.
Nigerian President-elect Muhammadu Buhari, due to take office at the end of the month, damped hopes recently when he said the girls may never be found, though he vowed his government would do all it could.
"As much as I wish to, I cannot promise that we can find them," he said.
Nigeria's national security advisor, Sambo Dasuki, said last week that Boko Haram would be cleared from the forest before Buhari takes office. But the government has a record of declaring military deadlines that are not met; for example, officials vowed to crush Boko Haram and capture its leader before the March 28 presidential election.
An expert on Boko Haram, Yan St-Pierre of the Berlin-based security analysis group MOSECON, said Nigeria's military had announced its planned attack on the forest 10 days in advance. That gave Boko Haram had the chance to spirit away its leader, Abubakar Shekau, other key figures and hostages, he said.
"We know they have tunnels in the Sambisa Forest," St-Pierre said. "It's a big place. Boko Haram has been there for a while. They've had time to set up camps, build tunnels, booby trap the area, set up fortifications and make it very difficult for anybody wanting to attack them.
"Boko Haram has set itself up properly," he said. "It's a big problem."
"They have returned to classic guerrilla tactics; they have spread their forces out. If you are going not going to bomb the whole area, you need very precise, surgical attacks," St.-Pierre said.
Nnamdi Obasi, an analyst on Nigeria with the International Crisis Group, said driving Boko Haram fighters out of the forest would be much more complex and difficult than retaking towns and villages.
"It's a vast area. It's rugged terrain. It's dense foliage," he said. "This looks like the last major camp of the insurgents. If the military is able to overrun it, it will be a significant achievement.
"It's going to take a longer time, more effort and perhaps more cost in terms of lives, but it's doable."
In recent weeks, Boko Haram abandoned Gwoza and other towns and villages, rather than staying to fight, in some cases returning later to mount attacks on the soldiers who had moved in. But while weakened, Boko Haram is far from defeated, according to analysts.
Its loss of territory signals an end to the group's effort to try to run a state and control a vast reach of Nigerian territory, but its members are likely to melt into the landscape, mounting guerrilla attacks and suicide bombings, as other terrorist groups have done after losing ground, analysts said.
"They have proven to be a very resilient and adaptable creature in the past," said Virginia Comolli of the London-based International Institute for Strategic Studies. "They've always been able to adapt their tactics in order to catch the military off-guard.
"I think it will be possible to push the group underground, but I think we are still going to see attacks, maybe over a smaller area," she said. "They might try to carry out a big attack to tell the world they're still strong and relevant."
Nigerian generals and politicians aim to capture or kill Shekau, the group's leader, but Obasi said even if Nigeria's military overran Boko Haram's base in the forest and managed to eliminate Shekau, it wouldn't spell an end to Boko Haram, which recently allied with the extremist group
"They will dissolve into cells and operate from various communities in the northeast, and they will retain the capacity to strike with lone wolf attacks and suicide bombings," Obasi said.
St-Pierre said support and funding from Islamic State could help Boko Haram set up cells in neighboring countries, after other small militant cells in the region have tilted toward Islamic State. He said there were several Islamic State-linked figures in the region, providing logistical and financial support to Boko Haram and other cells.
"The north-south axis to Libya is basically an open highway right now. It's one of the main supply routes. It allows [Islamic State] to supply Boko Haram," St-Pierre said.
Comolli said Boko Haram's alliance with Islamic State, at a time when the Nigerian militants have lost towns and territory, "was a way to say, 'We are still relevant, we want to be allied with the most dangerous 'successful' extremist group in the world. We are part of that club.'"