When news came in that a US Chinook helicopter had crashed in Afghanistan's eastern province of Konar, US officials must have known they were in for a search and rescue operation that would be the stuff of nightmares.
The Chinook plays a vital role in US military operations
It took the powerful US forces more than 24 hours to get to the wreckage.
The bodies of 16 US soldiers have been recovered after the Taleban say they shot the Chinook down.
"The terrain is unbelievably rugged. We are having to move everything there by foot and also using mules," Captain Michael Semmens, who is playing a key role in co-ordinating the operation, told the BBC.
"It's just unbelievable trying to get supplies in. Coupling the terrain with the enemy threat, these are very adverse conditions."
This is indeed the ideal territory for the Taleban and their allies to operate in.
Not only is the area mountainous, it is also covered with forests and dotted with narrow valleys, providing excellent cover for the insurgents.
The Governor of Konar province, Asadullah Waffa, told the BBC: "Even if you put hundreds of soldiers there you would not be able to find anything. The weather keeps on changing all the time."
"Our forces are working very hard as well. We have set up check posts in the province checking all the suspicious vehicles and individuals," Mr Waffa said.
Mr Waffa is the latest Afghan to say in public that the militants are operating from bases across the border in Pakistan, a subject of hot dispute between the two countries.
"These attacks are carried out mostly by foreign terrorist groups - Arabs, Chechens and Pakistanis.
"They come from the other side of the border, launch their attacks and cross back into Pakistan within 30 minutes.
"We have intelligence reports that Arabs and other terrorists entered Konar province from Pakistan in small groups with the help of Taleban and [its allies] Hizbi Islami loyalists," he said.
Konar province has a history of resistance.
Konar governor Asadullah Waffa says militants cross from Pakistan
It was one of the first provinces to rise against the occupation of Afghanistan by the Soviet Union.
Soviet and Afghan forces suffered heavy casualties there.
It then became the first province to fall to the mujahideen rebels.
Arab and other foreign fighters have had bases there for a long time.
The Arab influence is very strong with Arabs intermarrying with locals since the days of the Soviet occupation and also building their own mosques.
Konar province has one of the biggest followings in Afghanistan of the hardline Wahabi sect of Islam.
It feels remote from the capital, Kabul. There are few schools or hospitals and no good roads.
Most young people go to study in Islamic schools in Pakistan.