The Bacanga State Park protects a large area of mangrove and Atlantic Forest on the southern part of the island of São Luís, bordering the Bacanga River estuary. The mangroves here are among the most intact in northeastern Brazil, and the park is important habitat for migratory birds as well as caimans, monkeys, and other wildlife that still survives in the urban sprawl.
Visiting the park requires a bit of organization — there's no proper visitor infrastructure, and you'll want a guide or at least local knowledge for the forest trails. The mangrove areas are accessible by boat and offer a surreal experience of being in dense, tangled coastal forest within sight of the city's industrial port. Contact the Maranhão state environment secretariat for current access information.