Zona Sur. For the truly rugged adventurer, Osa Peninsula and Golfo Dulce in the southern region, known as Zona Sur, on Costa Rica's southwestern tip, is the country's most primitive and undeveloped terrain. Just getting here is difficult; many of the roads are little more than rutted, unpaved byways, with unpredictable river crossings and long-forgotten, overgrown paths. A bastion of tropical wilderness bordered by a pristine, craggy coastline, the region is largely made up of protected reserves and parks, including Corcovado National Park, Costa Rica's largest park and home to nearly 10 percent of all mammals found in the Americas. Given just a smattering of overnight accommodations, nature lodges are the best choice since they can organize guides for jungle treks, climbing expeditions, kayak tours and snorkeling trips.