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The solid material is denser than air. It does, however, form a structure which when evacuated of air has a lower average density than air.

Letting the air flow in fills in those gaps, raising the interstitial density enough that the average is greater than air.

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I keep reading this, but I'm missing what makes it special. If you create a large enough vessel of anything and evacuate all the air, you can make it lighter than air. Is it just that it's rare to find a material strong enough with respect to its density to withstand the necessary vacuum? (ie: would a titanium bubble with a sufficient volume to thickness ratio to be lighter than air implode? Or would it just be too expensive?)

Pretty much, though it's unclear to me whether this material is even strong enough for that. But make it big and strong enough, and you can make even a lead balloon float.

Buckminster Fuller thought that if you built a large enough geodesic dome out of normal midcentury materials, you could get enough lift from sunlight-driven thermal expansion of air to float it. http://www.thirteen.org/bucky/cities.html

> It does, however, form a structure which when evacuated of air has a lower average density than air.

Yet apparently it collapses at that point. Unless that's solved we might as well be talking about birthday balloons here.

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