Nicaragua’s second-largest tract of intact primary forest, the 2606-sq-km Reserva Biológica Indio-Maíz, lies about 15 minutes downriver from El Castillo. Thanks to a combination of geographic isolation and inaccessibility, the reserve was spared the chainsaw under Somoza, and once the Sandinistas took power it was legislated as a reserve and has remained off-limits to tourists.
Tragically a devastating forest fire in April 2018 destroyed over 50 sq km of forest, in what has been termed Nicaragua's worst-ever environmental disaster. To add insult to injury, Costa Rica's attempt to send backup firefighters and equipment was rejected by the Nicaraguan government, the cause of great anger locally.
Today the vast majority of the reserve remains restricted, but it's possible to visit some small designated sections where you can hike among 50m-tall, 500-year-old cathedral trees, search for brightly colored tree frogs and watch monkeys leap through the canopy.