"What the hell was that?" Connelly asked.

"I, uh, I have no idea." Robert scratched his head. "It was as though the ocean was full of sound. On earth you might expect something like this only in highly dense populations of vocal mammals, whales, dolphins, seals, what have you."

"Could BUD be malfunctioning?"

Robert's aghast expression said it all.

"OK, if BUD is working, what was that?"

Robert's hand froze on his chin as he rubbed his scruffy beard. He began making adjustments to the BUD system.

"What are you doing?"

"Reducing the radius."

"To what?"

"One hundred yards."

Robert knew it was a long shot, and he could feel Kathy's incredulous look on the back of his neck. If something were within one hundred yards, they should have seen at least a hint of it before. But as far as their eyes could see, nothing was there. Robert reminded himself: this is an alien world. It will be nothing like my preconceptions.

Or perhaps they were overlooking the obvious. The sounds could be coming from several sources. Water flowing through the tubes of ice above them, pushing pockets of gas toward the surface. The ice itself, pulled by the gravitational force of Jupiter, could be scraping as massive shelves of ice flexed. Thermal vents from the stone core could be spewing fluids and gases noisily into the ocean. All three scenarios fit the bill and all were more plausible than alien life forms.

Still, he couldn't keep himself from hoping.

After making the final adjustments to BUD, Robert looked back at Kathy. She wrung her hands together and nodded. Robert pushed the final key.

The sound was crystal clear, but in contrast to their last experience, dead silent. Robert and Connelly stood quiet and still, as though they had just turned to stone under Medusa's gaze.

Click, click, click.

The noise was faint, but clear and crisp. Three clicks that sounded like cupped hands clapping.

Connelly shot Robert a glance. He could read the expression on her face. What was that?

Robert had no answer and chose to remain silent in case it repeated.

Ten seconds passed.

A vibration filled the inside of the sphere that began at the lowest pitch the human ear could perceive and built higher and higher. It passed in and through their bodies, shaking their very bones. But it wasn't painful. The experience was almost pleasant.

The low tone was followed by four more clicks, three base thumps and another, less powerful tone, which sounded as though it origins differed from the first.

"I can't believe I'm hearing this," Robert said

Connelly pressed herself against the glass wall and tried desperately to see through the murky water. "Dammit! I can't see a thing!"

Three high pitched squeaks zipped through the sphere, followed by two thumps and another deep tone, which lingered and faded peacefully. Robert felt his body relax as the tone faded, taking his tension with it.