The Llanura de San Carlos comprises the easternmost part of the northern lowlands . The Ríos San Carlos, Sarapiquí, and others snake across the landscape, vast sections of which are waterlogged for much of the year.
The region today is dependent on the banana industry that extends eastward almost the whole way to the Caribbean in a gridwork maze of dirt roads and rail tracks linking towns.
Fortunately, swaths of rainforest still stretch north to the Río San Juan, linking Braulio Carrillo National Park  with the rainforests of the Nicaraguan lowlands, much of which is protected within private reserves.
Fishing is good, and there are crocodiles and river turtles, plus sloths, monkeys, and superb birdlife to see while traveling on the rivers. Even manatees have been seen in the lagoons between the Río San Carlos and Río Sarapiquí.
There are two routes to Puerto Viejo from San José , forming a loop ringing Braulio Carrillo National Park. The eastern route traverses the saddle between Barva Volcano  and Irazú Volcano  via Highway 32 (Guápiles Highway), dropping down through Braulio Carrillo National Park then north via Las Horquetas.
The less trafficked western route is via Vara Blanca, between the saddle of Poás Volcano  and Barva Volcano, then dropping down to San Miguel, La Virgen, and Chilamate. On January 8, 2009, this route was rendered impassable by massive landslides caused by an earthquake. As of 2011, it is now passable, however the makeshift route remains an unstable hair-raiser, subject to severe landslides.
A 4WD vehicle is recommended; drive at your own risk. Buses from San José to Puerto Viejo were not taking the Vara Blanca route at press time.