Pororoca is a bore in the Amazon river that could be the world’s longest natural wave.

“Twice a year, between the months of February and March, the Atlantic Ocean waters roll up the Amazon river, in Brazil, generating the longest wave on the Earth. The phenomenon, known as the Pororoca, is caused by the tides of the Atlantic Ocean wich meet the mouth of the river. This tidal bore generates waves up to 12 feet high which can last for over half an hour.”

Check out this great post on Fogonazos. It’s got some great pics, video and links.
Thanks, Fogonazos!