Oil-well fires have raged for months near Mosul, Iraq, and US soldiers in the region are choking on the toxic smoke and haze.
ISIS chose Mosul as its stronghold in the country about two years ago and have prepared for an assault ever since, according to Peter Bergen at CNN.
But as part of more recent efforts to slow the advance of Iraqi and Kurdish forces backed by a US-led coalition, which aims to take back the city and nearby towns and villages, ISIS militants torched oil wells and a sulfur mine near Qayyarah, located about 40 miles south of Mosul.
The plumes of fumes are so large that NASA can see them spreading into nearby countries from space. Meanwhile, soldiers and civilians on the ground are breathing it in. (Although the US has no ground troops in the Mosul advance, they are stationed nearby to provide air and intelligence support.)
Bill Duncan, a US soldier stationed in the region, has been photographing the scene from his base and posting them to National Geographic's "Your Shot" photography site. Duncan gave Business Insider permission to republish his photos here, along with captions he wrote, yet declined to comment further on his situation.
Here are some of Duncan's arresting photos, plus other images from war photographers and satellites that reveal what Duncan describes as a problem that could "haunt us for years to come."