Serra do Cachimbo is not a dramatic range. Its altitudes climb from 250 meters to a little over 500, modest by any continental standard. What it lacks in height it makes up for in character: a long southwest-aligned massif broken by flat-bottomed valleys and sudden escarpments, where the rivers tumble over edges in a sequence of rapids and waterfalls. The Salto do Curuá is one of them, a drop along the transition to the peripheral depression of southern Pará. Erosion has spent millions of years carving ridges and ravines into the Brazilian Shield here, sculpting what geology textbooks would call a dissected plateau and what pilots would simply call rough country. The range runs through four municipalities - Altamira, Itaituba, Jacareacanga, and Novo Progresso - names that read like a map of Amazonian frontier towns.

Surrounded by Amazon rainforest, the Serra do Cachimbo does something unexpected: it holds patches of cerrado vegetation at its higher elevations, a savanna biome stranded among the rainforest like islands of grass. That edge between ecosystems is why more than 380 bird species have been recorded here. The rivers that rise on the range - the Curua, the Jamanxim, and their cousins - hold fish found nowhere else on Earth. Some of these endemic species are threatened by habitat loss and proposed dams. One small fish, Phycocharax rasbora, has done the opposite: it thrives in the reservoirs that human engineering has created, a rare ecological winner in a landscape full of losers. The Nascentes da Serra do Cachimbo Biological Reserve protects part of the range, while Rio Novo National Park covers the southwest, where a sharp escarpment forms the northern face of the range.

At the eastern end of the range, surrounded by forest and far from any public road, lies Campo de Provas Brigadeiro Velloso - a sprawling testing ground administered by the Brazilian Air Force. It is one of the largest military complexes in the country, and its purpose has always been deliberately obscured. The complex includes Cachimbo Airport, which serves as the entry and exit point for whatever the military chooses to test in the Amazon's green privacy. For decades, rumors circulated that Brazil's nuclear program had used the site. The remote setting that makes this range a paradise for endemic fish and birds also makes it an ideal place to do things the government prefers the public not watch.

The Iriri River, born on these slopes, is the main watercourse of the Terra do Meio Ecological Station - a protected area spanning 3.37 million hectares. Terra do Meio means 'middle land,' and that is exactly what it is: a vast interfluve between the Xingu and the Tapajós, one of the least disturbed tracts of Amazon rainforest remaining. What happens on the Serra do Cachimbo matters far downstream. Dams proposed along these rivers would reshape fisheries 800 kilometers away. The logging and gold mining creeping up the frontier roads of Pará would poison the headwaters of an entire drainage system. From the air, the Serra do Cachimbo looks almost unremarkable - a long, low wrinkle in the green. But every drop that falls on it goes somewhere that matters.