The idea of being invisible has captivated scientists and comic-book writers alike for many years. As Einstein told us, it’s going to be pretty hard to do. But it’s somewhat easier to make something invisible to sound waves. Researchers in Spain have developed  a material that directs sound waves around an object so that they re-form on the opposite side with no degradation. Of course the military would  be interested in the material; your submarine would be invisible to Sonar if it used such a material,