And this time there was a real echo, very faint and far away. Perhaps it was from one of the many reflecting layers, deeper in this stratified atmosphere, perhaps it was another, more distant source. Falcon waited for a second echo, but it never came.

Mission Control reacted quickly and asked him to drop another probe at once. With two microphones operating, it would be possible to find the aproximate location of the sources. Oddly enough, none of Kon-Tiki’s own external mikes could detect anything except wind noises. The boomings, hatever they were, must have been trapped and channelled beneath an mospheric reflecting layer far below.

They were coming, it was soon discovered, from a cluster of sources about twelve hundred miles away. The distance gave no indication of their wer, in Earth’s oceans, quite feeble sounds could travel equally far. And for the obvious assumption that living creatures were responsible, the Exobiobogist quickly ruled that out.

“I’ll be very disappointed,” said Dr Brenner, “if there are no microanisms or plants there. But nothing like animals, because there’s no free oxygen. All biochemical reactions on Jupiter must be low-energy ones, there’s just no way an active creature could generate enough power to function.”

Falcon wondered if this was true, he had heard the argument before, and reserved judgment.

“In any case, continued Brenner, “some of those sound waves are a hundred yards long! Even an animal as big as a whale couldn’t produce them. They must have a natural origin.”

Yes, that seemed plausible, and probably the physicists would be able to me up with an explanation. What would a blind alien make, Falcon wondered, of the sounds he might hear when standing beside a stormy sea, a geyser, or a volcano, or a waterfall? He might well attribute them to a huge beast.