Ganymede said, “Pauline, is everything going well?”

“All is well,” said a woman’s voice, apparently from within the walls of the vessel.

“How soon will it be before we reach the ocean?”

“If we maintain this speed, it will be thirty minutes.”

“Is the Ariadne thread unspooling cleanly?”

“Yes.”

Ganymede said to Galileo, “The Ariadne thread is also a heating element, and will keep the central line of our flue liquid, to ease our return.”

They waited, absorbed in their thoughts. The light downward pull of Europa made the crew’s movements around the bridge fluid and slow, like dancing in a dream. Galileo found it hard to keep his balance; it was somewhat like floating in a river.

He drifted to Hera’s side and said, “All these machines have to work for us to stay alive.”

“Yes, that’s right.”

“It seems risky.”

“It is. But because it is, we engineer for safety. Materials and power are terrifically advanced compared to your time. And there is a principle called redundancy at the criticalities. Do you know this term? Replacement systems are available in case of failures. Bad things still sometimes happen. But there you are. They do anywhere.”

“But on Earth,” Galileo objected, “in the open air, the things you make don’t have to work for you to survive.”

“Don’t they? Your clothing, your language, your weapons? They all have to work for you to stay alive, right? We are poor forked worms in this world. Only our technologies, and our teamwork, allow us to survive.”

Galileo pursed his lips. There might be some truth to what she had said, but still he felt it obscured a real difference. “Worm or not,” he said, and she was a rather magnificently shaped worm, he did not add, “you could stay alive on Earth by breathing, eating, and staying warm. Granted these take effort, but you could make the effort. You have tools to help you, but they don’t have to remain unbroken for you to survive. A single man alone on an island could do it. There are no mechanical contrivances that surround you and protect you, like a fortress, that have to function successfully forever or else you very quickly die.”

She shook her head. “It’s like a sea voyage. You could not have your ship sink and survive.”

“But you people never land. You sail on forever.”

“Yes, that’s true. But it’s true for everyone, always.”

Galileo recalled standing in his garden at night, in the open air, under the stars. It was an experience this woman had never had. Possibly she could not imagine it. Possibly she had no idea what he was talking about. “You don’t know what it is to be free,” he said, surprised. “You don’t know what it is to stand free in the open air.”

She shook her head impatiently. “Have it your way.”

“I will.”

Again her amused glance, as if she were looking down on a child. She said, “You were famous for that, as I recall. Until things went wrong.”

The voice Pauline announced they were coming to the bottom of the ice layer, and were in what she called brash ice. They could hear floating chunks and clinkers striking the hull—a grinding noise full of scrapes and thuds.

Then they were moving freely, in water. Galileo had spent so much time on barges and ferries, and on a few well-remembered trips out into the Adriatic, that he recognized the feel in his feet. Such kinetic sensations were so slight as to disappear when one focused on them, but when focusing attention elsewhere, one became aware of the totality of the effect.

Ganymede said, “Pauline, search for the Europans’ flue, also any other vessels, of course. And give us an analysis of the water too, please.”

Pauline reported the water was nearly pure, with trace amounts of salts, floating particulates, and dissolved gases. Some of the crew began tapping madly at their desktops. Outside the window, the omnipresent blue had long since turned black. They might as well have been deep in the bowels of the Earth. Only one’s sense of movement suggested they were in a liquid.

Thus it was a great surprise to see a brief flash of cobalt blue in the window, like the random blue spark one sometimes saw crossing the inside of the eyelid.

“What was that!” Galileo said.

“We call that Cherenkov radiation,” Ganymede said.

“Somebody’s patron?” Galileo inquired, glancing at Hera.

“The discoverer of the phenomenon,” she said firmly.

Ganymede ignored their fencing. “There are tiny particles called neutrinos, which pour through our manifold in great numbers, but very seldom interact with anything. Once in a while one hits a proton—which is a small but substantial part of an atom—hits a proton in such a way that the proton releases a muon, which is a very small component of a proton. If that happens in an ocean like this, the muon will fly through the water in such a way as to spark a short trail of light in the blue wavelength. We will see a few per minute.”

Another little flare of blue appeared, again like the flaws that plagued Galileo’s vision. “Like shooting stars,” he noted.

“Yes. A very subtle fire.”

“A fire in water?”

“Well, a light, let us say. Though some fires will burn in water, of course.”

Galileo tried to imagine that. This dream was testing him in all sorts of ways. Could he find a way to test it back? Maybe answer the basic question: Was this really happening? He looked around to see if there was something small that he could take and conceal in his coat. Stealing ideas from dreams—perhaps it wasn’t so unusual. Perhaps it was a fundamental mode of thought.

The next flick of blue light was followed by a blue ball, which rapidly expanded, then became a kind of diffuse polyhedron, shedding spicules and other radiola of blue light that then curved away from the polyhedron in spirals—some of them tight equable spirals, making cylindrical coils, others equiangular spirals, growing wildly outward in conic shapes. One of these flashed right by the window, and for a second or two their chamber pulsed sapphire.

Some of the crew cried out, then there was silence.

Galileo said, “What was that?”

Ganymede appeared astonished. He stood pressed against the window, his blade of a nose touching it.

He straightened up, expression black, “It’s here. I knew it. The anomalies made it very clear. I’ve been saying so all along.” He turned to his crew. “We shouldn’t be here! Have the Europans shown up yet?”

“We haven’t seen them,” one replied.

“Find their flue, then! Get to it—we have to get to it before they do, to stop them!”

They turned back to their screens and their crowded desktops. After a time one said, “We’ve found it. They’re descending. We’re closing on it—wait. There they are. Two of them, just leaving their flue.”

Ganymede hissed. “Go!” he exclaimed. “Ram them! Get under them and ram them from below! Full speed until you reach them, then get in position to shove them right back up the flue!” He looked stricken, grim beyond telling. “We have to make them leave.”

“How can you do that?” Hera asked.

“We’ll ram them until they turn back.”

“Are you going to warn them?”

“I don’t want to break radio silence. Who knows what effect it might have on what’s in here?”

“What about the sound of collisions? What about the sounds and the exhaust from your engines?”

“That’s what I’ve been saying to them! None of us should be here.”

Another blue conic spiral flashed by them. Ganymede read the screens and the desks. “That could be some kind of signal. Speech, or thought, in some language of light.”

“Who would it speak to?”

“The light may be secondary. Who knows who it talks to? I have my suspicions, but …”

“Try numbers,” Galileo suggested. “Display a triangle, see if it knows the Pythagorean theorem.”

Ganymede shook his head, visibly trying to remain patient. “That’s what the Europans will do, I’m afraid. Reckless interventions like that. They have no idea what they may be getting into.”