Each year in February and March adventurous surfers gather in Sao Domingos do Capim, Brazil, for a chance to ride up the Amazon River on one of the most intense tidal bores in the world. Known as pororoca, the phenomenon occurs where the Atlantic and Amazon converge, sending a milk chocolate wave more than 12 feet high and crashing miles inland. Surfers who catch this wave need both skill and endurance as they’re in for a 7-mile ride upstream lasting 30 minutes or more. The name “pororoca” likely comes from the native Tupi word meaning “big roar.” And you will hear the roar of the wave building up well before you see it going up the Amazon at 20 mph, picking up trees, debris, and speed as it goes. Check out this wild water wall below.

While it doesn’t arrive at the same time like clockwork, the pororoca is predictable enough that the National Pororoca Surfing Championship is held each year. Image by Serginho Laus

Pororoca waves have been spotted as far as 180 miles inland. Image by Serginho Laus

The murky, chocolate brown Amazon can be a bit strange for surfers used to blue water and sandy shores. Image by Serginho Laus

The first recorded sightings of this wave date back to the 16th century. Image by Pororoca Surf Trips

The longest recorded ride on a pororoca wave lasted 43 minutes. Image by Pororoca Surf Trips

Amazon tidal bore waves as high as 30 feet have been recorded. Image by Pororoca Surf Trips

Tidal bore waves like the pororoca occur in about 100 rivers worldwide. Image by Serginho Laus

The powerful wave uproots trees, drowns wildlife, and carries upstream anything that gets in its path, all of which becomes debris for daring surfers to dodge. Image by Serginho Laus