The reserve lies along the Jutai River, a tributary of the Solimoes that drains northeast through Amazonas state. The Bia and Mutum rivers run through it as well, each feeding the Jutai from different directions. This is a transitional zone where several protected areas knit together into something larger than any one of them. To the west, the Vale do Javari Indigenous Territory - home to some of the last uncontacted peoples on Earth. To the east, the Rio Bia Indigenous Territory, which abuts the Uacari Sustainable Development Reserve and the Medio Jurua Extractive Reserve beyond. Cujubim is a piece of the Central Amazon Biodiversity Corridor, a mosaic of conservation units on both sides of the Solimoes. The terrain is flat, topping out at just 70 meters above sea level, and the air holds 2,460 millimeters of rainfall a year on average.

The story of who lives here begins in the 1940s. During World War II, after Japan captured the rubber plantations of Southeast Asia, the Allies needed Amazonian latex badly. Brazil and the United States launched a recruitment drive in the drought-stricken northeast, and tens of thousands of men - called soldados da borracha, rubber soldiers - were shipped up the rivers to tap wild rubber trees for the war effort. Many died of malaria, snakebite, starvation. Many never went home. Most of the families in Cujubim today descend from those wartime tappers. After the war the forest economy collapsed, and in the 1980s the lack of schools and clinics drove many families into the Amazonian cities. Some could not adapt to urban life and came back. When the reserve was surveyed before its creation, about 56 percent of residents could not read, and almost no one had more than a fourth-grade education.

The reserve sits within the Inambari area of endemism - one of the most biologically diverse zones in the entire Amazon. Researchers have recorded over 700 species of plants, and biologists estimate at least 600 bird species, 90 bat species, and 16 primate species inhabit the forest, though the area remains undersurveyed. The list of threatened and endangered mammals reads like an Amazonian wish list: giant otter, South American tapir, jaguar, cougar, and the Amazonian manatee, which drifts through the seasonally flooded varzea forests along the riverbanks. In 2015 researchers recorded the first Brazilian sighting of the eastern lowland olingo inside the reserve. The rivers hold pirarucu - one of the largest freshwater fish in the world - along with six-tubercled and big-headed Amazon River turtles, and white-lipped peccary herds range through the terra firma forest where the land lifts above the floodwater.

Residents fish, hunt, farm small plots, and extract from the forest - oils, straw, vines, fruits, honey, and rubber. The most valuable timber species include Ceiba pentandra, copaiba, Virola, Calophyllum brasiliense, Ocotea cymbarum, Carapa, and Cedrela odorata, the last of which is the cedar most prized by furniture makers. The main cash income comes from salmorado fish - particularly surubi - along with logging and turtle capture. For generations the economics ran through regatoes, the river middlemen who bought forest products cheaply and sold city supplies dearly, keeping many families in perpetual debt. The reserve's management program has worked to break that cycle through direct credit and marketing. The agencies pushing alternatives to logging now promote copaiba oil, andiroba oil, and rubber, along with fruit, vegetables, and medicinal plants - the traditional knowledge of a forest economy that long predates any reserve boundary.

Cujubim became a reserve on September 5, 2003, created by decree 23.724 of the state of Amazonas. The deliberative council formed in April 2008, the management plan was approved in March 2009, and support has come through the Amazon Region Protected Areas Program - the international conservation partnership launched in 2002. A sustainable development reserve is a particular Brazilian innovation: not a park that locks people out, but a protected area designed around the traditional populations already living inside it. At Cujubim, that means 56 families, 36 households enrolled in the Bolsa Floresta forest-protection payments program, and a single enormous forest that is at once the largest such reserve on the planet and one of the most thinly populated. From cruise altitude, it is an uninterrupted green.