“That was when I saw them falling, the ones closest to the water. Just falling down as if they’d decided to go to sleep.”

“And you ran toward them,” I said.

“I was responsible for them. What else could I do? I’d only run a few paces before I could smell that rotten-egg stink—”

“Methane?”

“Yes. And then I understood what had happened.”

It had been a “methane burp.” He told me that deep under the Arctic sea floor there are vast reservoirs of trapped gas. Molecules of carbon dioxide, hydrogen sulphide, and methane can be trapped within cages of water-ice crystals — ice formed under extreme conditions of pressure, under the weight of the sea. You can find such stuff in sediments all the way around both poles, immense banks of ice and compressed gas. There is thought to be as much carbon locked up in these reservoirs as in all the world’s fossil fuel stores. Until that dreadful day in Siberia, I had never even heard of them.

And it’s very compressed, at more than a hundred times atmospheric pressure. Any engineer would recognize it’s not too stable a situation. When the “lid” is taken off that pressure vessel — for instance when the permafrost starts to melt, the containing pressure relieved — the eruption can be severe.

I thought it through. “So a pocket of these gas hydrates gave away. The carbon dioxide and methane came gushing up. Carbon dioxide is heavier than air, so it would settle back to the surface of the sea and start spreading out…” Choking anything in its path.

Everybody understood the consequences of a carbon dioxide flood. There had been an incident on Cephalonia ten years earlier that had killed thousands, an industrial accident, a carbon-sequestration scheme gone wrong.

All of a sudden Tom broke down. He buried his face in his hands. “I couldn’t get them all out. The stink of the methane drove me back. And I was scared, scared of the cee-oh-two. I couldn’t help them.”

I couldn’t even touch him. I had to sit and watch, frozen, as the competent soldier put her arms around his shoulders. “You couldn’t have done any more,” Sonia said. “Believe me, I saw your medical charts. You went as far as you could.”

“Well, one thing’s for sure,” I said. “You can’t stay here anymore.”

Tom looked up, and anger flared in his tear-streaked face. “You always said I was a quitter, didn’t you, Dad? I’m not going anywhere.”

As I tried to work out what to say, Sonia butted in. “Actually Mr. Poole’s correct. The aid agencies won’t support any more work in this area, Tom. You’re going to have to leave. We can’t get you directly back to the States. But we can chopper you to Moscow, then to a military base near Berlin, and then by civilian charter to London.”

I kept my mouth shut, knowing from long experience that while he might listen to Sonia, he certainly wouldn’t listen to me.

At length Tom said miserably, “All right. But the sequencing project—”

“That will go on,” Sonia said brightly. “They have robots to do this sort of thing now.” She stood up. “I’ll make the arrangements. I’ll, umm, I’ll leave you to it.”

She pushed her way out of the tent, and Tom and I were left together, inarticulate, joined by electronics, separated by more than distance. We started to make plans. I would fly to England to meet him in person, if I could.

But even as we talked it through I was thinking over what had happened here, and in a corner of my mind I wondered what would happen if all those icy methane deposits, all around the poles of the planet, decided to yield up their treasures in one mighty global burp.

On the ocean world, in the shelter of the flitter, Reath continued Alia’s education.

“Have you thought any more about what I’ve told you?”

“You haven’t told me anything,” she said sourly. “Nothing but that list of names. The Implications of This and That—”

He laughed. “Of Indefinite Longevity. Of Unmediated Communication. Of Emergent Consciousness.”

“What am I supposed to think? They’re just names!”

“Isn’t that enough? Alia, do you imagine I have a set of textbooks for you? To become a Transcendent is a process of discovery about yourself.”

“You mean I have to figure it all out?”

“You may discover more wisdom in yourself than you imagine. Let’s start with the first stage, for instance—”

“Indefinite Longevity.”

“What do you imagine that means?”

She thought about it. “Not dying.”

“Why is this important?”

“Because the Transcendence was created by immortals.” Every child knew that, even though it sounded like nothing but a scary story.

“Do you think extreme longevity is possible?”

She shrugged. “On the Nord we expect to live to five hundred or so, barring accidents. In Michael Poole’s day, it was rare to live much beyond one hundred. Surely it would be possible to come up with a treatment to stop the aging process altogether.”

“An immortality pill?”

“Yes.”

“And if I had such a pill, and gave it to you, you would expect to live forever?”

“Not forever. There will always be accidents. This stupid platform might fly up and tip me off into the sea any second.”

He laughed. “Yes. Undying, then, if not immortal. But, statistically, with luck, you could expect a much longer life span. An indefinite life span, in fact.”

“Indefinite Longevity.”

He smiled. “You see, we don’t just pluck these terms out of the air. And how would that make you feel?”

“It would be a wonderful gift. So much extra life—”

“Don’t spout clichйs, child,” he said.

She was taken aback; he rarely snapped at her.

“Think it through,” he said. “Suppose it were true. How would you feel?”

To know you would certainly die one day was one thing. To know that there was at least a chance that you might live on and on and on, without limit, would change everything. How would she feel? “Different.”

“How? What about other people? You’ve just had an almighty row with your family. Would you feel differently about that if you thought that you might face millennia more of life?”

“I wouldn’t have had the row at all,” she said immediately. If her mother were to die before they could be reconciled, Alia would always regret it. And if she lived for tens of millennia or more, that regret would burn away at her soul, irresolvable. “It would drive me crazy, in the end. If I knew I was not going to die I’d try not to do anything I might have to regret forever.”

“You’d become cautious.”

“I wouldn’t make enemies. And I wouldn’t hurt my friends.” But I might not even make friends, she thought, if I knew I might be stuck with them forever — or, worse still, outlive them.

Reath was watching her, as if trying to follow her thoughts. “What else? I know you are a Skimmer. I envy you that! But the real excitement of Skimming comes from the risk, doesn’t it? Now, as things stand, if you were to have an accident, if you managed to kill yourself, you would be giving up a few centuries of life. But what if you were risking millennia — an indefinite future?”

She snorted. “I’d have so much more to lose. You don’t think about that consciously when you Skim, but — if I took your pill I’d never leave my room!”

“Suppose your sister was here with us now, and she fell into the sea. Would you try to rescue her?”

“Yes.”

“You’d risk your own life to save hers?”

“Yes!”

“Even at the cost of a hundred thousand years of existence?”

“I…” She shook her head.

“How do you think other people would feel about you?”

“They would hate me,” she said immediately. “They would envy me — turn against me.”

“For your long life? Even if they knew that your longevity was for a purpose, for their own betterment?”

“Even so. Nobody would see past the fact that I would live on when they were dust. I would think like that. I would have to hide…” She shook her head. “Some gift it would be! I’d be paralyzed by the thought of all that future. I’d have to hide away.”