Sunken ships and strange ocean changes presage the mightiest and most unaccountable threat ever made against this world of ours. The sinister menace suddenly overshadows the activities of all mankind in the year 1990. As a defensive move three intrepid souls--Jeff Grant, Arturo Plantet and Tad Megan--boldly plunge into a world beneath the Pacific. Down beneath the ocean's floor Jeff Grant and his companions struggle against the mysterious, infernal power of mankind's enemies as the threatened cataclysm comes ever nearer. With the oceans drying and the surface world disrupted, humanity has not long to wait for the hellish attack of the Gians--and for the terrific climax of this assault from the sea. Out of the depths comes the white aëro, gliding serenely over the Earth, not laden with death-dealing bombs or weapons of military warfare, but with something far more terrible--disease germs! With complete annihilation of the surface peoples impending, the inhuman, unreasoning Gians swarm out from their undersea abode. Out of the depths comes the Crimson Rainbow, laden with the hate of centuries, fuming and boiling with a heat that melts the very mountains! Is it a life-saver or is it the seal of doom? Is it the hope of the human race or is it but another instrument of torture? Topping all the blood-curdling thrills of the entire story is this smashing, crashing climax which it provides.