WARNING: Graphic images
They called it Willie Pete during WWI and WWII but the innocent name conceals a far more sinister reality.
This is a weapon of war favoured by those aiming to inflict the most misery on their targets.
Its signature is a loud bang and a plume of bright white smoke, but its effects are felt most when long white streams carrying chemicals start falling from the sky.
Those within the fallout zone will suffer such severe burns that if they ever recover they'll be scarred or crippled for life. Victims have recounted being burned through flesh and bone.
Willie Pete's other name is WP, or white phosphorus, and it's being used again after years on the sidelines.
In Iraq, where Islamic State and a US-led coalition are locked in a tense stand-off, white plumes are again filling the air. Experts say those fleeing the conflict are the most at risk. Human rights groups say use of the weapon is bordering on a war crime.
Her oxygen mask started melting
A New York Times photographer captured an image last week of smoke above a town near Karemlesh, east of the Iraqi city of Mosul.
Amnesty International reports the dispersal pattern is consistent with a US-made white phosphorus munition capable of spreading the deadly chemicals over an area between 125-250m wide.
It's not clear who fired it because US-led forces, Peshmerga forces and Iraqi central government forces are all taking the fight to ISIS there. Whoever it is, human rights groups say it has to stop.
"White phosphorus can cause horrific injuries, burning deep into the muscle and bone," Donatella Rovera, Senior Crisis Response Adviser at Amnesty International, said.
"It is possible that some of it will only partially burn and could then reignite weeks after being deployed."
That's what happened to an eight-year-old girl named Razia who was badly burned when a white phosphorus shell landed on her home in June, 2009.
Human Rights Watch reported that as doctors began treating her, the oxygen mask on her face began to melt. They said flames appeared when doctors began scraping at dead tissue.
Pictures of Razia show her crying out in pain with burns across most of her face. She survived, and she's one of the lucky ones.
Grey area surrounding white phosphorus
White phosphorus - also used in rat poison - is so revered because it can lay dormant in the ground, on clothing and even on skin. When the chemical is exposed to air, it immediately ignites.
There's a big problem with banning it, too. The use of incendiary weapons against civilians in general is illegal and has been since signatories declared it so in the 1980 Convention on Certain Conventional Weapons Protocol III. But the protocol's definition of an incendiary weapon does not specifically refer to white phosphorus bombs.
The reason for that is white phosphorus has other valuable uses: militaries around the world use it to create a smoke screen and to mark targets, but it's not yet clear why a bomb was spotted over Mosul.
The US has previously condemned the use of WP. In 2009, weeks after Razia was badly burned, a spokeswoman for the US Army said white phosphorus was being used against US troops in Afghanistan.
"This pattern of irresponsible and indiscriminate use of white phosphorus by insurgents is reprehensible and should be noted by the international human rights community," Major Jenny Willis said.
She said the weapon could cause "unnecessary suffering", as defined in the laws of warfare.
WARNING: Graphic image below
Afghanistan, Israel, Syria, Iraq
It was used by Israel during the Gaza conflict prior to 2009. A report, titled "Rain of Fire", documented witness accounts of 22 days of constant shelling between December 27, 2008 and January 18, 2009.
One man told Human Rights Watch he saw a victim stumbling in an alley. When he got closer, he saw first hand the effects of the chemical weapon.
"My daughter told me there was a car on fire with people in it," he said.
"I looked out and saw a young man who had lost control of himself trying to push his way into the burning car. When I got to the car he had fallen down and he was on fire.
"The shelling was ongoing and I dragged him to an alley and tried to talk to him, but he couldn't talk. One of his eyes had burned away and he was horribly injured."
Russia was accused of using the controversial weapon in 2015 in Aleppo, Syria, a city besieged by ongoing war.
Karemlesh is the latest city under fire. Amnesty's Ms Rovera said civilians there are at risk in the coming days and weeks.
"It is absolutely imperative that the forces using white phosphorus publicise details of areas potentially contaminated by the substance," she said.
"Such information is also crucial for medical professionals operating in Iraq so that they are aware of the kind of injuries they are treating. Tragically we witnessed people dying in Gaza because doctors were not aware that their patients' burns were caused by white phosphorus and were thus not able to dispense the right treatment, resulting in the wounds deteriorating."
More than 10,000 people have been displaced from Mosul but it's believed more than 1.5 million remain trapped inside the city and on its outskirts.