Breakthrough material barely denser than air - CBS News: Researchers at HRL Laboratories and the Composites Center at the University of Southern California have created what they say is the lowest-density material, a lattice of hollow tubes of the metal nickel.
Its volume is 99.99 percent air, and its density is 0.9 milligram per cubic centimeter--not including the air in or between its tubes. That density is less than one-thousandth that of water.