Beachgoers in Portugal scrambled for cover over the weekend as the sweltering heat gave birth to a rare “tsunami roll,” an 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.

Rare roll cloud phenomenon advances over the ocean like a giant wave, captivating onlookers and spreading rapidly online.

A rare roll cloud was seen passing over several beaches in Portugal on Sunday, June 29. The phenomenon features a cloud that strongly resembles a tsunami quickly rolling over a space where two contrasting temperatures merge.