Canopus (Canopus #3)

Fiery exploding skulls; super volcanic eruptions; interstellar travel; a mission gone horribly wrong; the ground turned to liquid; a miraculous launch; trapped half a mile beneath the surface of the planet and surrounded by otherworldly fire - the situation is looking dire for Helen and Arther as they attempt to blast their way back to the surface, Helen learns the truth behind the mission to Canopus amping up her urgency to return home.