After colonial plantations decimated the forest, an ambitious rewilding program aims to ensure the survival of Rio de Janeiro’s ancient rainforest.

One minute, I’m choked by fumes under a concrete overpass. Minutes later, I’m soaked in sweat, sticky with the respiration of a forest. That’s not something you feel in every city – especially one with more than six million inhabitants. Rio de Janeiro might be known for its beaches, football, and Carnival, but few realize it contains the world’s largest urban forest: the Tijuca forest.

Tijuca is not a typical forest. First protected in 1861 – a decade before the first national park in the United States—Tijuca forest is a 40 sq km chunk of Atlantic Forest, a once-vast biome that covered 1,000,000 sq km of Brazilian coastline. Today, roughly 15% of the Atlantic Forest remains decimated by sugarcane and coffee plantations and logging by the European colonists who first stepped on Brazil’s shores in the 16th Century.

Losing the Atlantic Forest over the next 200 years was nearly a death knell for Rio de Janeiro’s young settlement. Rivers that quenched the city dried up, and drought was imminent. Nineteenth-century Emperor Peter II had a solution: bring back the forest. So, in the 1860s, farmers and city dwellers living on forest land were expropriated, and enslaved Africans were ordered to plant more than 100,000 trees. However, they didn’t bring back many animal species that once thrived in the forest.

In 1967, the Tijuca forest was declared a national park divided into three non-contiguous sectors: Tijuca Forest west of the city center; Carioca, where the world Wonder Cristo Redentor statue is located; and Pedra Bonita and Gávea mountains overlooking the beach. Tijuca National Park has since become the country’s most visited, with more than 3.5 million visitors annually.

“Picture in your mind just for a second Rio without the forest, just the bare mountains and the seas – Rio would not be the wonderful city anymore,” said Fernando Fernandez, a professor of ecology at the Federal University of Rio de Janeiro and co-founder of the conservation organization Project Refauna. But the Tijuca forest isn’t just eye candy – it’s a critical temperature regulator and source of freshwater for Brazil’s second-largest metropolis. And according to Fernandez, the forest isn’t healthy.

When you hike through Tijuca National Park, the forest looks like it’s thriving. Trails leading to over 30 waterfalls and climbs up iconic mountains like Pedra da Gávea and Pedra Bonita reveal lush flora with drooping vines, giant jackfruit splattered on the ground, and cute capuchin monkeys picking bananas apart. But Fernandez says the forest’s tall trees are slowly dying and not being replaced.