Peat swamp forests form where waterlogged soil prevents dead vegetation from fully decomposing. Leaves fall, branches snap, trees topple -- and instead of rotting, this organic material compresses into thick, acidic beds of peat that can reach 20 meters deep, with pH levels as low as 2.9. The water that pools among the trees is stained dark brown by tannins leaching from the peat, giving these ecosystems their local name: blackwater swamps. On Borneo, peat swamp forests occupy the coastal lowlands of Indonesian Kalimantan, Malaysian Sarawak, and Brunei's Belait District, built up behind brackish mangrove zones and bounded by better-drained lowland rainforest. Most are geologically young, less than 5,000 years old, but the peat itself is a carbon archive -- each meter representing centuries of stored atmospheric carbon. About 62 percent of the world's tropical peatlands lie in the Indo-Malayan region, with Indonesia alone holding 80 percent.

These are not the bleak, mossy bogs of northern Europe. Borneo's peat swamp forests support trees up to 70 meters tall, rooted in substrate so spongy and unstable that they require buttresses, stilt roots, and pneumatophores just to stay upright. Despite the extreme acidity and low nutrients, 927 species of flowering plants and ferns have been recorded in these forests -- more than three times the 260 species found in a comparable study in Peninsular Malaysia. The vegetation arranges itself in concentric circles from swamp center to fringe, with species composition shifting as water depth and peat chemistry change. Many of the tree families are shared with the surrounding dipterocarp forests, but numerous species are found nowhere else. Root mats concentrate in the upper 50 centimeters of peat, where the trees can access what little oxygen and nutrients the waterlogged ground offers. Protected areas shelter fragments of this biodiversity: Tanjung Puting and Sabangau National Parks are the largest, alongside Brunei's Belait Peat Swamp and portions of parks spanning Sarawak and Kalimantan.

In the mid-1990s, the Indonesian government launched the Mega Rice Project, draining over one million hectares of peat swamp forest in Kalimantan for conversion to rice paddies. Between 1996 and 1998, workers dug more than 4,000 kilometers of drainage and irrigation channels. The plan backfired. Instead of irrigating the land, the channels drained the peat, dropping water tables and leaving the forest surface dry year-round where it had previously flooded two meters deep during the monsoon. Roads and railways built for legal forestry opened the region to illegal loggers who stripped nearly all marketable timber. Forest cover in the project area dropped from 64.8 percent in 1991 to 45.7 percent by 2000. The Indonesian government eventually abandoned the project, but the damage was structural: once drained, peat does not simply re-wet. The channels had turned a carbon sink into a fire hazard.

When the drained peatlands caught fire during the El Nino droughts of 1997-1998 and again in 2002-2003, the consequences reached far beyond Borneo. Dense smoke and haze blanketed Southeast Asia for months, causing respiratory problems among half a million people. Satellite imagery from the European Space Agency showed the smoke plumes stretching across the South China Sea. Scientists calculated that 91.5 percent of the 790,000 hectares that burned in 1997 was peatland, not surface forest. The peat combustion alone released an estimated 0.19 to 0.23 gigatons of carbon, with another 0.05 gigatons from the vegetation above. Fire damaged nature reserves like Muara Kaman, destroyed new agricultural plantings alongside old forest, and killed uncounted birds, animals, and reptiles. The fires returned in 2006 and 2009, each time filling the skies with the acrid evidence of an ecosystem that, once destabilized, keeps burning. Indonesia became the world's third-largest carbon emitter, driven substantially by the destruction of forests that had been quietly storing carbon for millennia.