NAZARÉ, Portugal — The big ones typically come only once a year, in the winter. Whipped up by powerful storms in the North Atlantic, they roll for days toward Europe, rising to seemingly impossible heights before crashing on the shores of Praia do Norte, a beach along the Portuguese coast just north of this picturesque fishing town.

“It is like an earthquake,” said Pedro Pisco, a city hall administrator from this old fishing port, a few miles away from Praia do Norte. “When it breaks, you can feel the earth shaking under your feet.”

The area has a reputation as a dangerous spot for its turbulent gales, crushing surf and frequent accidents, though on a recent day the sea was flat as a reflecting pool. Normally, though, the waves crash on shore with a special power, and for years residents were not even sure they were safe to surf.