The idea of indestructible killer robots may sound like something straight out of the Terminator movie. But they could soon become a reality, as scientists have just witnessed metal healing itself for the first time, without any human intervention. A US-based study has overturned everything we thought we knew about metals by revealing that cracks from wear and tear can actually mend themselves under certain conditions. It's a discovery that has the potential to revolutionise engineering, with the prospect of self-healing engines, planes and even robots now on the horizon. 'This was absolutely stunning to watch first-hand,' said Brad Boyce, a scientist at Sandia National Laboratories who led the study with Texas A&M University.