Look down, not up. In Indonesian Borneo, the most dangerous fires are invisible. They burn underground, deep inside a 4.5-million-hectare belt of tropical peatland — one of the largest such ecosystems on Earth — quietly consuming carbon that has been locked away for thousands of years.

Above ground, the sky over Banjarbaru hangs amber and thick, like tea that has steeped too long. The haze has already crossed international borders, reaching Singapore, Manila and Kuala Lumpur in what experts are calling the region's worst transnational smog crisis in over a decade. Volunteer firefighter Dwi Sujatmoko, 35, has been breathing this air for more than a month. "It hits your nose straight away, your eyes burn — that's more or less what it's like," he says.

He and thousands of other volunteers — many of them indigenous Dayak and Banjar people — are battling an inferno that has already scorched some 200,000 hectares of forest and farmland across the Indonesian portion of the island since early July. They carry hand tools, carts and portable pumps against an enemy that hides below their feet. "If underground fire spreads and can no longer be controlled," Sujatmoko warns, "the losses are enormous." What is making this crisis especially alarming for climate scientists is not the flames themselves but what the flames are unlocking.

When peatlands burn, they release millions of tonnes of carbon dioxide, carbon monoxide and methane — greenhouse gases that had been safely stored for millennia, now suddenly free. A so-called Super El Niño has intensified Indonesia's drought conditions dramatically, multiplying active fire hotspots across the country.