Putting problems with containing hydrogen aside, it seems this would actually work. A structure made with 99.99% hydrogen (0.09 kg&#x2F;m3) and 0.01% aluminum (2700 kg&#x2F;m3) by volume would have an average density of 0.36 kg&#x2F;m3. This is lower than air (1.29 kg&#x2F;m3) and bit more than twice the density of helium.