The Ring of Fire — Peter R. Stone

Bradley Millner, Megan, and their little Mechanical friends, have a problem. They completed the mission the Orb gave them, but instead of taking them home, it dumped them onto an oil rig. An oil rig that's drilled a well deep into the belly of the Earth. Normally that's not a problem, but it is when there's an evil Mechanical onboard that's planning to drop a powerful bomb down the well. A bomb that will cause over four hundred volcanoes in the Ring of Fire to erupt, and trigger massive earthquakes and tidal waves. Actually, there's another problem. Brad and his companions have less than an hour to foil the evil Mechanical's diabolical plans, but there's no sign of the mechanical monster. Try as they might, they can't find where it's hiding. And they can't stop it if they can't find it. Join Brad and his friends as they team up with some new companions and try to thwart the evil Mechanical's plans.