Spartacus - and the Sun Beneath the Sea

Spartacus and the Sun Beneath the Sea was a French animated series (original title: Les mondes engloutis). Two siblings, Bob and Rebecca, travel down to the center of the earth where they discover a civilization. This civilization escaped from the great cataclysm, and was not aware of the people living on earth's crust. They have their own sun, Shagma, which is ill, and Bob and Rebecca must assist them.