Complex sentence: Instead of exploding on the surface these missiles would burrow down through the planet until they reached the core where they would explode, sterilizing the surface of the planet with the resulting seismic upheavals and atmospheric contamination.
Split into simpler sentences: Instead of exploding on the surface, these missiles would burrow down through the planet until they reached the core where they would explode. This would sterilize the surface of the planet with the resulting seismic upheavals and atmospheric contamination.