Inhumanoids (1986)

Acting on information forced from Blackthorne, the Inhumanoids raid a Soviet air base, stealing a handful of nuclear missiles. A seemingly repentant Blackthorne warns Earth Corps that the missiles are intended to shatter the divide between the Earth's crust and mantle, but upon travelling into the Earth to preclude this catastrophe, the team discovers that Blackthorne has set them up and that the missiles are actually meant to detonate the planet's core. Earth Corps is able to convince D'Compose to allow them access to Infernac — a deal made easier by the fact that the Inhumanoid already fears for his own survival in the face of Metlar's insane plan — and they manage to defuse enough of the missiles to thwart the explosive scheme.