Here's an idea that might be just barely possible now with improved materials:  Visualize a hollow spherical shell in earth's atmosphere at sea level enclosing a volume from which most of the gas(air) has been removed.  Further imagine that the hollow spherical shell is A)light enough to be neutrally bouyant and B) strong enough not to collapse under the approximately 14.7 pounds per square inch imposed on it by earth's atmosphere at sea level.  It can be shown that  Density(shell)/Density(air)= 1/(1-inner radius^3/outer radius^3).  This expression addresses the density of the shell and not its strength.  Such a low pressure balloon could be an interesting technology demonstrator for strong, lightweight materials. Fabrication would be a frustrating exercise, no doubt. Bill