The teddy bear, garish but cuddly, is propped on top of the explosives it was designed to hide and detonate. An adult would probably have walked by, but to a child the wide eyes and fuzzy orange fur would have been irresistible.

“Why would Isis use something nice, like a bear or a rabbit? They used this toy because they know the peshmerga [Kurdish fighters] will not touch it, but children will,” said Colonel Nawzad Kamil Hassan, an engineer with the Kurdish forces, who says his unit has cleared more than 50 tonnes of explosives from areas once controlled by the militants.

As a broad coalition of forces tries to push Isis out of Mosul, its last major stronghold in Iraq, Hassan has decided to preserve some of the most creative, cruel and unusual of those homemade bombs to use as training aids for new recruits to one of the most dangerous jobs in the world. The homemade explosive devices provide a lesson in the depths of ingenuity, intelligence and resources that Isis devotes to spreading murder and fear even when its fighters can no longer terrorise in person.

In the areas where it rules for long enough to seed them with bombs, the group has created a dark, parallel universe, where even the most mundane object can kill. A toy, a playing card and an abandoned watch are all detonators designed to spark the acquisitive curiosity of a returning civilian, who would be maimed or murdered by the explosion.

An ordinary hose lying across a road is another simple but ingenious detonator. A bundle of old clothes, which a dog or cat could step across without harm, would have exploded if someone had picked it up to reclaim or throw away. A pile of mud and stones is a concealed mortar. A discarded piece of plywood would have activated a bomb when it was picked up or kicked aside, as a ball bearing rolled down a tube to complete the circuit. Duct tape, a lever and a trip wire turn a door into a deadly weapon.

Even a simple playing card is used by Isil to attract someone to an anti-personal device Photograph: Alessandro Rota/The Observer

The expertise of Isis with explosives and extensive use of suicide bombs and booby traps makes attacking them particularly dangerous, and slows recovery when they are defeated. Because there is no area, civilian or military, that the group will not lace with explosives, many Iraqis fear that returning to their homes and former lives could be deadly, even long after militants have left.

In Sinjar city, a centre for the Yazidi minority who have been victims of some of Isis’s worst atrocities, Hassan’s men cleared five tonnes of explosives from a single school, he says. His engineers’ unit, 1st Support Force Command, has been working through areas to the west of Mosul city for around two years.

They must second-guess every pile of stones or rubble, every door and window, every apparently discarded or abandoned object, on guard against their own complacency as much as the bombs themselves.

“Every day there is a new device. Some of our men have disposed of things that others have not even seen before,” he says. His men have little protective equipment. Two have been killed and 15 injured. The training school, set up by Hassan’s unit with help from the French military, is an attempt to limit that toll, both by teaching recruits to recognise and defuse Isis bombs, and by giving them a sense of the capacities of the forces they will fight.

Their war is often a shadowy battle of wits, as they try to find and defuse devices planted weeks, even months, earlier. But the engineers also provide mine-clearance for advancing troops, when they sometimes face Isis and its bomb-makers in close contact.

At the entrance to the first room in Hassan’s school, there are two of those “fighters” waiting in suicide vests and explosive backpacks. They provide a moment of visceral shock. In fact, they are only shop mannequins, but the bombs they wear are real, taken from injured fighters. The balaclava covering one plastic face makes it less obviously incongruous, more disturbingly realistic. Beside them hangs an Isis flag, seized in a cleared area, and a glass frame with rows of detonators and rockets improvised from gas canisters and barrels.

Then the rows of different bombs begin: there are the cleverly camouflaged ones, intended to take out a single victim, then others designed to maim and kill more widely. Some are designed for active combat and must be detonated by a fighter close by, such as the trio of radio-controlled pressure cooker bombs that could be used in sequence, first to maim a group of attackers, then to take out their rescuers. “They are set up with a view to secondary explosions,” Hassan explains. Others are controlled by mobile phones or wires.

They are not even animals, they are worse than animals Colonel Nawzad Kamil Hassan

Perhaps the majority of the bombs are created to lie in wait for their victims. The most frequently used pressure-plate IEDs (improvised explosive devices) are so common that hundreds are dumped in a pile at the entrance to the classrooms. To an untrained eye, thin and wrapped in a tangle of wires, they look like the discarded fittings for strip lighting. Others are formed from pressure cookers and ball bearings, fibreboard and bedsprings, jugs of explosives and trip wires. The threat is in open spaces as well as in buildings and on roads.

Wires, painted green and arranged through grass, would have detonated at a single touch. The mortars, smeared with mud and pebbled with fake stones, had been placed on a dusty, windy plain where nature would conspire to hide them further. A few of the devices target the engineers themselves, who are a particular enemy for Isis because of their work to spearhead ground operations and undo its deadly traps in liberated areas.

Hassan points to a small, easily detectable mine that is actually a booby trap for a well-buried pressure-plate IED. “So when he goes in to defuse this, he will step on the other and it will explode,” he explains.

The collection grows with almost every mission, and Hassan fears it will not stop expanding until Isis is deprived of the space and resources to think up new ways of delivering death.

“They are not even animals,” he says, picking up the deactivated teddy bear in disgust. “They are worse than animals.”