Pristine graphene — a microscopic sheet of carbon atoms arranged in a honeycomb pattern — is among the most impermeable materials ever discovered, making the substance ideal as a barrier film. But the material may not be as impenetrable as scientists have thought. Researchers have found that the material bears intrinsic defects, or holes in its atom-sized armor. The results point to the possibility of promising applications, such as membranes that filter microscopic contaminants from water, or that separate specific types of molecules from biological samples.