The beast that river-dwellers have long feared is coming later this week.

It’s a wave called Pororoca, the great roar, and its low bellow can be heard up to an hour before it arrives. It tumbles in from the ocean and travels up the Amazon on a path of destruction. While the region’s residents accept the river’s wrath and start moving their boats and animals, daredevils called bore riders grab their surfboards.

This beast that they will ride is called a tidal bore, a wave that flows in from the ocean and propagates to dozens of rivers around the world. Along the Amazon, one of the strongest bore tides brings big waves that travel for miles and seem to last forever, perfect for a long ride.

Local legend has it that three mischievous children travel up the Amazon playing practical jokes. But the scientific harbinger of this curse — or gift if you’re a surfer — is that familiar force we call gravity.