(CNN) Oil wells, deliberately set on fire by ISIS, have turned skies black in and around Mosul, Iraq's second-largest city.
The terror group's scorched-earth tactics are reminiscent of the 1991 Gulf War, when Saddam Hussein's forces ignited hundreds of oil wells as they retreated from Kuwait.
Photographer Sebastiao Salgado was in Kuwait back then as oil workers fought the flames and eventually stopped the spill, the largest in world history. It took nearly an entire year.
"Moving like phantoms through the gloom, covered in oil for hours on end, these men were too close to danger to think of anything but the job at hand," wrote Salgado, who documented the disaster in his new book, "Kuwait: A Desert on Fire." "It demanded experience, improvisation, discipline, solidarity and immense toughness of body and mind. Without them, the environmental and human cost of this calamity would have been immeasurably greater."
Salgado dedicated his book to the "fearless men" who risked their lives to end the environmental crisis. He said they came from many different parts of the world, enduring "unbelievably difficult circumstances" to extinguish the fires and cap the oil wells.
His photos show their faces splattered with oil as they scramble to stop gushing wellheads. Nearby, massive walls of fire reach several stories high.
Salgado recalls intense heat, toxic fumes and deafening noise.
"By the end of each day, my jaws literally ached from the sheer tension of being exposed for hours on end to heat, noise and oil and to the perennial hazard of a major explosion," he said.
Because many wellheads were damaged, they had to be repaired or replaced, Salgado said. That required metal tools, and when using them there was always the threat of a spark causing another fire.
"It was a bit like trying to put a new faucet on a broken water pipe -- without turning off the water," he said.
Salgado remembers seeing workers become dizzy or faint while working in the field. Their colleagues would stop and drag them to safety.
"Here more than ever I saw the importance of both their professionalism and team spirit: the men looked after each other in the knowledge that, if they were in trouble, they too would be cared for," Salgado said.
While Salgado spent each night in a hotel about 30 miles away from the Ahmadi oil fields, the men lived in camps. They used a mixture of gasoline and detergent to wash themselves in their own water lake, the photographer said.
Salgado was on assignment for The New York Times Magazine, but many of his photos from Kuwait were never published. Looking through them 25 years later, he felt they had a timeless quality.
"It was a voyage into the past," he said. "I relived the moments I took them and I was as moved by what I saw as I had been a quarter of a century earlier.
"Never before or since have I witnessed an unnatural disaster on such a scale."