This idea was posted prior to Monday, October 30, 2006.

Consider Styrofoam.  It's light and strong.  The small, spherical cells that make up the substance are filled with gas, presumably atmosphere, at ambient atmospheric pressure.  Suppose that the cells were, instead, evacuated.  That is, suppose that they were (if I may use such an irrational phrase) filed with vacuum.  The small size and spherical shape of the cells would give a chunk of such material sufficient structural strength to resist crushing at atmospheric pressure.  It would be buoyant in atmosphere.  The number of uses for such a material are limited only by imagination.  Consider, for example, the savings in weight if all insulation and light duty structural material in airplanes was buoyant.

How to make the stuff?  I'd say that it would be simple to make it in orbit.  Vacufoam production should be a dandy space industry.  Of course, you'd have to weigh the stuff down to get it to the surface.