The second-largest tract of intact primary forest in the country, the Reserva Biológica Indio-Maíz is a vast wilderness and a hugely important rainforest ecosystem with incredible biodiversity. While much of the reserve is restricted, it is possible to visit certain parts of it. In 2018 horrific fires caused by outsiders, who were clearing forest illegally to use as farmland, destroyed huge areas of ancient forest and it will take decades for the reserve to recover. At the confluence of Río Bartola and Río San Juan, the park ranger's office administers a 3km walking trail that winds through the reserve's towering trees. A local guide is obligatory to enter and most visitors come on a package from El Castillo. Another 20 minutes downriver by boat is the reserve entrance at Aguas Frescas, where the scenery is similar but there is a slightly longer and more challenging trail. While Aguas Frescas was once less visited and hence attracted more wildlife than the Bartola section, both trails are now very popular and choosing a section depends mainly on how far you want to walk. Note that despite the collection of entrance fees to use the facilities, both trails are in very poor shape, with deep mud the norm in many parts, so bring sensible footwear and wear long pants. About an hour further along from the mouth of the Río Sarapiquí, the San Juan Delta begins to weave through the wetlands, meeting up with the almost-as-enormous Río Colorado. Birding becomes increasingly interesting, and fishing even better – but note that you have officially entered the bull sharks’ territory, so no swimming. When you finally enter the expansive Bahía de San Juan del Norte, you’ll notice the rusted old dredger owned by Cornelius Vanderbilt’s Transit Company, which kept the shipping lanes open for would-be gold prospectors en route to San Francisco. The dilapidated dock to the south marks the entrance to what’s left of Greytown, founded on what was then the mouth of the Río San Juan, now a sandy extension of dry land. After you cross the bay to the mouth of the remarkable Río Indio, you’ll reach San Juan de Nicaragua, where you can explore black-water creeks, hidden lagoons and thick jungle within the wide reach of the Indio-Maíz.