In the furnace of the Ifoghas mountains, a purgatory of dust and sharp black rock straddling the Mali-Algeria border, soldiers hunt down a ghost-like enemy, invisible but ever present.
This featureless region, sometimes known as the Adagh des Ifoghas, or the mountains of the Ifoghas tribe, has become the frontline of Operation Serval, France's mission to rid northern Mali of militant Islamist groups.
Some 1,200 French troops backed by a Chadian contingent are engaged in battle with insurgents deeply entrenched in the dessicated terrain of one of the most inhospitable places on the planet.
The soldiers have set up camp on a former army base in the riverside oasis of Tessalit, a Tuareg village which serves as a stop-off point for traders crossing the Sahara desert.
Their mission for today is a "cleaning" operation and they set out in the dead of night for an exhausting ride in armoured vehicles which takes several hours on a bumpy track.
Two tactical units are tasked with seizing one hill each in order to take control of the valley below.
Soldiers then meticulously rake the valley floor on foot for three days and two nights. There will be no shots and no direct contact with the enemy who, outnumbered and out-resourced, avoid hand-to-hand combat.
On a terrain lacking even the slightest shade and not unlike the surface of the moon, the soldiers advance methodically, each sweating silently in 50C (122F) heat under the burden of a helmet, bulletproof vest, heavy weapons and five litres (11 pints) of water.
The "shock troops" eat and speak little, drinking the bare minimum, enduring hours of extreme heat, flies, dust which gets everywhere, jammed weapons which constantly need cleaning, and long hours of marching or observation.
They sleep under the stars in the mountains with the temperature dropping to what ought to be a more comfortable 15C, but the marked difference from the daytime heat makes the troops shiver in their sleeping bags.
-- Hostages on their minds --
The enemy may be elusive but they leave behind plenty of clues that they have been nearby just moments earlier -- here a heap of combat clothing left in a hurry, there the casing of an anti-aircraft gun.
An oppressive quiet hangs over the Ifoghas mountains but everyone knows that the silence could be shattered at any moment by fresh fighting.
Islamists are past masters in the art of camouflage. They bury themselves in rocky enclaves, so well hidden that on one occasion a French patrol passed a few centimetres in front without discovering them, the soldiers say.
The question of French hostages in the hands of Islamist groups somewhere in the Malian desert is on everyone's mind and feeds the evening conversation in the camp.
In all 15 French nationals are being held captive in Africa, with Al-Qaeda in the Islamic Maghreb (AQIM) claiming responsibility for six of the kidnappings.
Frustratingly, it is impossible to know if any of the abductees are close by or hundreds of kilometres away.
Just days after the "cleaning" operation, AQIM claimed to have executed Philippe Verdon, a Frenchman seized from his hotel in northeastern Mali in November 2011, although the killing remained unconfirmed by Paris as of Thursday.
Explosions ring out constantly, whether it's the French blowing up a weapons cache or a suspicious pickup truck being destroyed by a delta-winged Mirage military plane.
A column of brown smoke rises in the distance.
The northern desert area comprising about 60 percent of Mali fell to ethnic Tuareg rebels following a military coup a year ago.
But they lost control to Al-Qaeda linked radicals who imposed a brutal version of Islamic law, carrying out amputations, executions and beatings, before France sent in troops and took back the cities of the north in January.
French losses remain low since the beginning of the intervention but danger is always present.
In the Ifoghas mountains on March 16 a new explosion punctures the silence, only this time the sound seems somehow different, and the troops' radios haven't announced any "friendly fire".
For the first time, Islamists have managed to blow up a tank, an improvised explosive device killing 24-year-old Corporal Alexander Van Dooren, the fifth French soldier to die in Mali.
In Tessalit just 100 soldiers, faces expressionless, attend Van Dooren's funeral, with most his comrades out in the field.
General Bernard Barrera, chief of the French land forces in Mali, delivers the tribute but there is no fanfare, not even a bugle.
A soldier begins The Marseillaise a capella, and the rest of the troop join in.
(A longer version of this report is available on AFP's Making Of blog at http://bit.ly/15pQdmS)