This is the lighter than air solid developed in the 1980s, based on agar, a derivative of seaweed.
During manufacture, the internal foam space can be filled with a variety of gasses, including helium or hydrogen and the material is then shrink wrapped to prevent leakage. This example probably contains ordinary air.
This video was recorded over a decade ago about 1997.
Updated versions of the material are produced in huge quantities for military purposes.
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