The Spruce Goose still amazes me.
Back in the days of sail, Navies found that there was a limit to the size of an all-wooden ship of the line, with the three-decked 100 gun ships like L'Orient being about the maximum size possible. Bigger than that, rigidity and strength began to diminish. Iron and steel finally permitted larger ships.
With the Goose, I am guessing that critical components (like the spar) are aluminum, and the fuselage likewise has metallic major supporting structures. Otherwise, it'd simply fall apart under its own weight when under load.