Beachgoers in Portugal scrambled for cover over the weekend as the sweltering heat gave birth to a rare “tsunami roll,” or ominous wave of dark clouds over the water. Viral video of Sunday’s phenomenon showed bathers in Praia de Buarcos in awe of the enormous roll cloud approaching the beach as it began blocking out the sun and kicking up gusts of wind.

Check out this incredible roll cloud spotted off the coast of Portugal on Sunday. Roll clouds are most common with thunderstorms, but can also form when cooler air over the ocean sweeps under hot air over land,