The United States may have forgotten how to make FOGBANK: a critical component in at least some thermonuclear weapons. FOGBANK is an ‘interstage material’ that gets turned into a superheated plasma by the detonation of the ‘primary’ fission bomb, helping to ignite the ‘secondary’ fusion reaction.
Some speculate that FOGBANK resembles aerogel. Others describe efforts to re-learn how to make it.