A thunderstorm that at first seemed unexceptional to scientists turned out to produce some of the biggest hailstones on record, which pummeled central Argentina in 2018.

Powerful thunderstorms moved across the city of Villa Carlos Paz in Córdoba province in February 2018. Matthew Kumjian at Pennsylvania State University in University Park and his colleagues collected photographs, videos and stories from residents about the incredible hail.

The biggest hailstone measured an estimated 18.8–23.7 centimetres wide. That’s potentially bigger than the 20-centimetre world record currently held by a hailstone that fell in South Dakota in 2010. The scientists propose that any hailstones measuring 15 centimetres — about the size of a honeydew melon — or more should be categorized as gargantuan.

As the storm developed, weather-radar images showed nothing in it that hinted that it might yield such enormous hailstones. The authors say that meteorologists should work closely with the public to document gargantuan hail, to understand the conditions that lead to it.