The rushing sound from underwater was a loud, throbbing hum which had no relationship with the humming sound that drove fish. Two spoutings of gas from mine-explosions shot up. There were more concussions in the water.
Then something broke surface. It was huge, and looked like a rocket. It leaped. No, it dashed upward, toward the sky. It flashed skyward, accelerating as it rose. Something else broke the surface and headed for the heavens. This one was globular.
There were dull concussions coming from far underwater, and more rockets broke surface and shot skyward.
Anti-aircraft guns were fired. Shell-bursts came close, but not close enough. Not less than twenty enormous rockets leaped out of the water and shot up toward the sky. Some observers claimed there were more than thirty. Down to southward, where the bathyscaphe had been crushed, the planes that were dropping mines reported that four other objects broke loose from the ocean and fled for empty space at speeds too great to be estimated.
Terry looked suddenly astonished.
"But ... of course!" he told Deirdre. "When you need high pressure, of course you've got a weakness. You can't take concussions! Anything underwater is completely vulnerable to bombs! Whatever was down there has found out that the natives—we aborigines—have a weapon they can't face. Primitive stuff. Explosives! Chemical explosives! And creatures that can travel between planets and undoubtedly have atomic power and—who knows what else—can't fight back if we drop submarine mines on them!"
A last object broke surface and hurtled skyward. Behind it, deep, deep down, there was a titanic explosion.
"Ah!" said Terry. "That was a time-bomb! They've gone home for good!"
A task force of a private yacht, a fishing boat, a satellite-tracking station, an airplane carrier and a mine layer had driven off an invasion of earth. But the public could not be told that the earth had been invaded. The people who had been involved in this secret adventure had to be satisfied with the realization that they had saved mankind.