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.

A rare tsunami-shaped cloud formation came rolling across the northern and central coast of the country, shocking crowds and prompting a flurry of photos and videos online. The phenomenon, occasionally referred to as a “roll cloud,” was seen billowing over the ocean as many on shore stopped what they were doing to film the unique sight.