Sebastian Steudtner looks like an ant compared with the giant wave he’s riding off the coast of Nazare, Portugal. The video served as a submission for a new world record — currently held by Rodrigo Koxa. His ride on an 80-foot wave granted him a spot in the Guinness Book.

Steudtner’s big day happened in January 2018, but for whatever reason (and do we need one?) the video of him flying down a wall of water as tall as an average office building went viral this week.

First reaction: How is this even possible? To say nothing of the physics of surfing a wave this large, how does one get to be 80 feet tall?

Outside of extreme surfing circles, Nazare isn’t a familiar name on the list of reliably big-wave beaches. Those places tend to be Mavericks in California, Jaws on the north coast of Maui or Waimea Bay. Nazare, though, has a very specific bathymetry that can generate some of the largest waves in the world — at least near the coast.

There’s a canyon under the water. It slices through the continental shelf from Nazare to the open Atlantic. As waves approach the Portugal coast, they are split in half because of the different sea floor depths. One side of the wave slows down and gradually gets taller as it runs into progressively shallower water. The other side approaches the shore faster in deeper water.

Eventually, the shape of the canyon forces the waves to rejoin, and the result is additive. In wave physics, it’s called constructive interference — two waves merge together to create one giant wave. (If you’re struggling to picture this, there are several videos to illustrate the exact process, including this one starting around 2:20.)

In general, waves tend to be larger in winter because of turbulent storms and stronger winds. According to the Inertia, a website devoted to surfing and other extreme outdoor sports, the people in Nazare “think that the bunch of wild men that show up every winter to surf there are insane.”

Correction: An earlier version of this story referred to a viral tweet that was incorrect. It showed Steudtner on the wave in 2018, but referred to it as Roxa’s record in the text. The story has been modified to reflect the facts, and the correct video from the World Surf League itself.