"Terahertz rays" may sound like something Captain Kirk ordered ready to fend off enemy craft, but they're real and they've got an intriguing use: discovering murals hidden beneath coats of plaster or paint in centuries-old buildings.

In a paper published in the February 15 issue of Optics Communications, a team of researchers from the Louvre Museum (paging Dan Brown!), Picometrix, and the University of Michigan used pulses of terahertz radiation ("T-rays") to detect colored paints and a graphite drawing of a butterfly through four millimeters of plaster. Next month, they're taking the show on the road to help archaeologists examine a mural that was recently discovered behind five layers of plaster in a 12th-century church in France.

What's so special about T-rays? Not only can they detect the faintest pencil sketches under paintings on canvas without harming the artwork (or the people using them--their radiation is non-ionizing), but they also trump existing imaging methods in their ability to detect art materials such as graphite and chalk with unprecedented depth and detail. Bianca Jackson, first author of the paper and a doctoral student in applied physics, explains it better and more spookily: "Terahertz is a strange range in the electromagnetic spectrum because it's quasi-optical," she says. "It is light, but it isn't." We do love a dubious glow!