It has been a couple years since Tom Swift solved the mystery and overcame the aftermath of a loss of helium in the undersea mines he once discovered. Since then not much of anything has happened that might be considered newsworthy in Helium City. In fact, he and his friend, Bud Barclay, haven't even traveled down there in all that time. Things are about to change A deep thrumming vibration begins to make itself noticed in Helium City. As it comes and goes with no indication of its origin or meaning, Tom designs and lowers a series of probes meant to discover what is coming from deep inside...