It seemed too good to be true, says Allard Mosk. It was 2007, and he was working with Ivo Vellekoop, a student in his group at the University of Twente in Enschede, the Netherlands, to shine a beam of visible light through a 'solid wall' — a glass slide covered with white paint — and then focus it on the other side. They did not have a particular application in mind. “I really just wanted to try this because it had never been done before,” Mosk says. And in truth, the two researchers did not expect to pick up much more than a faint blur.