EVERY profession has certain rules of thumb. Over the last few days in the devastated Iraqi city of Mosul, a bomb disposal expert told me of the all-important ones in his own line of work: “Presence of the abnormal, absence of the normal”, was how he summed up the tell-tale signs of booby-trap devices as he moved into liberated but still lethal terrain.

Anyone who has seen the critically acclaimed movie The Hurt Locker, about a US army explosive ordnance disposal (EOD) team during the Iraq War, will instantly recognise the hazardous job of such operators. As I listened to this man and his colleagues tell of the daily dangers they face, I couldn’t help thinking of the battle-ruined buildings in Mosul they had taken me to see just a short while before.

These are their places of work, just as the office, shop or university might be to most of us. Confronted with these shattered labyrinths of stone and concrete in Mosul’s streets, I found it nearly impossible to imagine how anyone might spot something out of the ordinary in the extraordinary chaos and destruction the war has created.

Only earlier this week US General Stephen Townsend, who heads the campaign against so-called Islamic State (IS), described the fighting in Mosul as “the most significant urban combat to take place since World War Two”. Certainly, the levels of physical devastation in some areas are reminiscent of that conflict.

Given this, what is becoming clearer by the day is that ordinary civilians are by far paying the heaviest price for the close-quarters fighting in Iraq’s second city. What’s more, long after IS are routed from their neighbourhoods they will continue to pay that toll.

The gun battles, mortar fire and airstrikes still rage in the west of the city. But those neighbourhoods liberated by the Iraqi Army have been left infected by a deadly contagion of improvised explosive devices (IEDs) the jihadists deliberately deployed on an industrial scale before their retreat.

For terrorist groups like IS, such devices are the weapons of choice. In 2015 alone IEDs, easily constructed, were responsible for killing nearly 18,000 people around the world and immobilising and posing a threat to millions more.

Those whose job it is to clear up these indiscriminate weapons say that what they are encountering in Mosul, is on a scale and level of sophistication rarely seen before. To put this into some kind of context, one EOD operator working for the British-based Optima Group, a company comprised mainly of former military ordnance disposal experts, told me a few days ago of what they had found and had to deal with in the Iraqi town of Ramadi when liberated from IS in 2016.

In the town’s university more than 3,000 explosive devices and booby traps had to be cleared in an operation that took a full year but was concentrated on a very small area. Ramadi is a fraction of the size of Mosul, as is its university. Earlier this week an EOD team from the Optima Group took me to the pancaked heaps of ruins that sit inside Mosul university campus. Driving toward the campus, we passed a huge crater in the road made only two days before after an IS suicide bomber detonated his explosive belt.

It was a sharp reminder to both civilians and those with the task of clearing IEDs in places like Mosul’s university grounds, schools, electricity sub-stations and water plants that IS may have been pushed back into the west of the city but its cells still operate in the east too. This makes for an environment terrifying for civilians, some of whom are trying to return to what remains of their homes from nearby camps for those displaced by the fighting.

For those EOD teams trying to “decontaminate” key buildings so that some semblance of infrastructure and basic facilities can function, it poses a continuing security threat.

Doug Napier, a former US Army Ranger who works for Optima Group in and around Mosul, says the scale of IS devices the the organisation is finding both in term of quantity and power is “phenomenal”.

Some explosive devices that are triggered by someone stepping on a pressure plate, he says, are sensitive enough to be detonated by a child but big enough to blow up a tank. Most of these countless IEDs have been manufactured on a factory scale using easily obtainable materials but constructed in a way that makes them horrendously effective.

So organised is IS’s production line that Mr Napier says he can look at the parts and recognise when they have come from the same manufactured batches.

“They even have their own quality control labels,” says Optima Group’s programme manager Charlie Martell, another former soldier who finds himself running a team of highly trained specialists around Mosul who have years of experience clearing landmines and IEDs from across sites in Iraq.

At the group’s base east of Mosul, Mr Napier showed me a small patch of training ground into which are sunk a variety of mock IEDs of the type they encounter regularly and which kill and maim Iraqi civilians every day.

Among them are plastic jerry cans full of ammonium nitrate and linked to detonators, pressure plates that are activated when trod on and even have anti-tilt and tamper mechanisms so that those trying to clear or render them safe will themselves activate the explosion.

Then there are crush wires, small strands of wire that have circuits that, when stood on, create a blast but are near impossible to see when strewn on the ground in the likes of Mosul’s rubble strewn streets.

All of these are totally indiscriminate weapons, killing children and adults, families returning to their homes or people trying to flee ongoing fighting.

The number of fatalities and casualties from these devices is growing daily and will continue to do so. As one EOD operator put it, the war of attrition against the civilian population that these bombs is responsible for is truly “horrific”.

“Go into a school that is functioning again and the walls are covered with the photographs of children who have been their victims,” one EOD operator told me.

While IS might now be on the back foot militarily in Mosul, its legacy is going to be felt for a long time to come. For, long into the foreseeable future, the “presence of the abnormal” in the shape of IED’s along with the appalling pain and suffering they cause is set to plague the people of Mosul.