Exploding boules balls

A local festival, a pleasant summer's evening, a friendly game of boules - and suddenly a deafening bang. Not from a firework or a cracker, but from an exploding boules ball. The incredible explosion, which occurred in Nettetal, Germany, on the Dutch border, left a hole in the tent and a crater in the ground. Fortunately, nobody was hurt in the incident; the revelers merely got the shock of their lives. But how can boules balls just blow up like that? Following similar cases in Switzerland in 2009, a team of Empa experts carried out an in-depth investigation ( Video in German). Their conclusion: the cheaply produced, thin-walled balls were filled with mortar to make them heavier. Unfortunately, the mortar was damp and contained iron filings, which corroded.