Rocky and Bullwinkle in Upsidaisium

Bullwinkle inherits a mine on Mount Flatten. When he and Rocky go there, they find that Mount Flatten floats in the sky because it is the only source of upsidaisium, the anti-gravity metal. Its value is such that Boris and Natasha's overlord, Mister Big, becomes personally involved in attempts to steal first the mountain and then the government's storehouse of ore.