The people of Valencia are still trying to come to terms with the events of 29 October - in a matter of hours, the Spanish city was hit by flash floods, leaving behind a trail of death and destruction. But, as news of the floods began circulating on social media, so did rumours about the supposed causes behind the torrential rain. “This is not normal weather”, suggested one tweet seen more than a million times, “This is weather warfare manipulated by HAARP.” Claims that the weather is being manipulated through the High-frequency Active Auroral Research Program (HAARP), an atmospheric research programme first developed by the US military, are not new. And yet, as many extreme weather events become more frequent and intense, these lies appear to be finding new audiences. So, what is the truth about HAARP?