KAREMLASH (Iraq) • "They're everywhere," said the Iraqi intelligence officer, sweeping his arm from this ancient Christian village towards the horizon.
The Iraqi captain was searching for tunnels dug by Islamic State in Iraq and Syria (ISIS) fighters.
He stomped on the ground. "Here. We found one, then three, now six. Right here." And over there? "More," he said. "And more."
Villages recaptured from ISIS over the past three weeks by the Kurdish peshmerga and Iraqi army forces on the road to Mosul have been honeycombed with tunnels, many of them booby-trapped.
In the past three days, commanders say Iraqi forces have faced the hardest fighting of the offensive as they entered Mosul, made worse by extensive tunnels that are allowing ISIS fighters to appear seemingly out of nowhere, attack, then retreat to the hidden bunkers.
"The clashes have been very, very violent", especially on Friday and Saturday, as troops advanced deeper into the city held by ISIS for the past two years, said Brigadier-General Yahya Rasoul, a spokesman for the Iraqi military.
It's like we are fighting two wars in two cities... There's the war on the streets and there is a whole city underground where they are hiding.
COLONEL FALAH AL-OBAIDI, of the Iraqi counter-terror forces, on the battle.
"It's house-to-house fighting now," he said, declining to give casualty figures.
With Iraqi special forces now battling for the eastern districts of Mosul, commanders say the omnipresent tunnels, alongside holes smashed between the walls of buildings, are allowing hidden ISIS fighters to move quickly into position to ambush advancing troops and then pivot to concealed locations.
An Iraqi armoured commander who drove his Abrams tank into eastern Mosul recalled seeing dozens of fighters scrambling on the street in front of his guns.
"Then they disappeared" into the ground, he said.
"It's like we are fighting two wars in two cities," said Colonel Falah Al-Obaidi of the Iraqi counter-terror forces. "There's the war on the streets and there is a whole city underground where they are hiding."
The colonel complained: "Now it's hard to consider an area liberated, because though we control the surface, ISIS will appear from under the ground, like rats."
Col Obaidi and other commanders said that they knew urban warfare among civilians and human shields in Mosul would be difficult, but the tunnels are making it worse. The officers describ1ed the battlefield as more of a sphere than a plane - with threats coming from side to side, above and below.
The number of tunnels is unknown. ISIS fighters dug extensive tunnels under the central city of Fallujah, which was recaptured by Iraqi forces in June.
ISIS didn't invent the tactic. Tunnels have been used in warfare for thousands of years, especially valued in asymmetric guerilla war.
Jewish rebels used tunnels against Roman legions; the Viet Cong did the same against American troops in South-east Asia.
In Karemlash, Iraqi Christian militias uncovered cramped earthen burrows used by ISIS fighters to hide from surveillance drones, artillery shells and US-led airstrikes.
The tunnels were dug into a hill that covers an archaeological site of an ancient Assyrian city.
ISIS also commandeered the Saint Barbara convent and dug deep tunnels through the floor of the chapel. The village marks the site where Alexander the Great fought the Persian emperor Darius in 331BC.
Some tunnels go for hundreds of metres. The earth is hard-packed and laced with rocks and the passageways illuminated with electric lights. Iraqi troops have found weapon caches, small kitchens, food pantries and rooms stacked with explosives.
To conceal their digging from drones and satellites, ISIS fighters usually hid the soil.
The longest tunnel discovered so far stretched for almost 10km at the edge of Mosul, according to Iraqi commanders, dug by ISIS cadres with help from civilians probably forced to shovel.
The tunnelers employed drills originally designed for mining operations or the oil fields, the Iraqis said.