DAMASCUS: Guided more by what they can hear than see, Syrian troops and rebels are battling in the bowels of Damascus, digging tunnels in a campaign to control the eastern entrance to the capital.
This is the suburb of Jobar, next to Abassid Square, a stronghold of President Bashar Assad's regime. The army knows that if it is lost to the rebels, the capital's entire defences are in danger.
Inside an empty building, a hole in the floor leads to an observation room 7 meters down, where army computers are linked to cameras rigged up inside a network of army tunnels.
But Mazen, a captain, explains that the cameras are of only limited use in tracking the enemy's movements.
"We depend above all on our ears. When we've located the source of the noise, we dig in that direction."
"Then it's a surprise: either the rebels are there and we fight, or we cut off the tunnel, or we use it ourselves."
There are two wars going on here, one above ground, the other below.
In the open air, snipers from both sides fire from buildings sometimes just metres apart, the thunder of shells and regime air strikes occasionally rocking the area.
"There are two cities. There's the virtual one above, and the real one below," says a soldier monitoring computer screens.
To evade the snipers, men on both sides have linked the buildings they control through a network of tunnels dug through the red earth and lighted with lamps.
"The first tunnel is for food supplies, the second to communicate between our positions and the third to evacuate the wounded," explains Maher, a bearded army officer in fatigues.
"The last, which can go to depths of 12 metres, surrounds the building and, if the enemy tries to get in, we detonate bombs placed inside it."
The two sides dig their tunnels at different levels to reduce the potentially deadly risk of bumping into each other, in what has become a game of cat and mouse.
"Captain Ali drives them crazy, because he knows how to dig and launch surprise attacks," says one of his men.
The army was forced to respond when its position in western Jobar came under attack last year, with rebels bombing some of the buildings it occupied.
"The tactics of the gunmen are twofold; to dig right up to our buildings and blow them up, or to drill tunnels passed our lines so they can get into the city behind our backs," says a local commander, Colonel Ramez.
Ramez has called in geologists equipped with sensors capable of detecting cavities at a depth of up to 15 metres, but sometimes they just find pipes dating back to Roman times.
For the army there are two red lines -- Abbasid Square and Jobar's March 8 Tower, which would allow rebel snipers to target much of eastern Damascus if captured.
Several months ago the army discovered a rebel tunnel that ran behind their lines, and managed to thwart "a massive attack on Damascus" at the last minute, the colonel says.
According to a Syrian intelligence officer, their plan was to send 30 suicide bombers to target several buildings, opening the way for 1,000 rebels to sow chaos in the army's ranks.
In a document entitled "War of the Tunnels", the press office of the Syrian Revolutionary Forces also said the tunnels had several purposes, including securing food supplies for besieged towns, reaching the enemy and destroying their positions.
"There has never in the world been such a thick network of tunnels as there is in Syria. It started in Homs in 2012, and the army has since discovered 500 of them. But I think there are twice as many," says Salim Harba, an academic in Damascus.