You may wish to look at our US patent application 20070001053 (11/517915). We propose an evacuated sandwich spherical shell with two thin face sheets and a light core between them. Finite element analysis confirmed that the structure using commercially available materials (e.g., boron carbide face sheets and aluminum honeycomb core) can be light enough to float in air and strong enough to withstand the atmospheric pressure with decent safety factors for strength, buckling, and intracell buckling. Actual manufacturing, while definitely possible, is not easy.
