Surfers brave the 'Pororoca', one of the world's most feared waves in the river Mearim in the Brazilian Amazon. Pororoca, which means 'great roar' in the indigenous Tupi Guarani language, is a phenomenon that occurs twice a year, when the ocean waters of the Atlantic meet the currents of the Amazonian rivers at high tide and push them in opposite directions. In Arari, a town of 30,000 residents in the state of Maranhao (northeastern Brazil), the freshwater wave usually manifests itself in March and September, during full and new moon days, as a brownish avalanche up to four metres high that travels twice a day along the Mearim River. IMAGES AND SOUNDBITES