Van Lyle stared at me. I think I had actually managed to surprise him.

“You’re asking me what a Geotron is?”

“I am.”

“But didn’t Professor McAndrew explain to you?”

“He would have done — if I had given him half a chance.”

“Well… I’m not a scientist, as you know very well.”

“Nor am I. That ought to make things easier for both of us.” We started walking toward a sleek high-speed aircar, as Van Lyle said, “Well, you know what neutrinos are, don’t you?”

“Yes. They’re elementary particles, with no charge, and a tiny rest mass. Their discovery was predicted by Pauli in 1931, because they were needed to preserve the laws of conservation of energy and angular momentum.”

That was gross intellectual dishonesty, and I knew it. But Lyle didn’t. He looked quite impressed.

“Right,” he said. “All they have are spin and energy. And they don’t interact much with ordinary matter, uness they have very high energy. That makes them the devil to detect. A free neutrino can easily pass right through the Earth. But sometimes that can be an advantage. Like if you want to do down-deep exploration. And you decide to build a Geotron.”

He explained the rest of it as we took off and he flew us west at Mach Ten. The staff of Earth’s Food and Energy Council had done all the easy exploration of Earth’s interior that they could do — which meant prospecting to about twenty kilometers down. Now they were forced to search deeper, or else be dependent on off-planet resources. The Geotron was nothing more than a huge kind of X-ray machine, for examining the inner structure of the Earth. But instead of X-ray radiation, which would penetrate no more than a few feet, the machine generated tight beams of high-energy neutrinos. They could be sent in any direction. They passed right through the middle of the Earth, scattering off structures in the interior, and emerged at points around the world where their numbers were measured. Then a very fancy set of computer programs took the information on the detected neutrinos and used that to deduce the interior structures that they had encountered in their path from the Geotron to the detection chambers.

“Looking for primordial methane, as the primary target,” Lyle explained. “Pockets of compressed methane left over from the time of the Earth’s formation, and still trapped deep inside.”

“To use as fuel?”

“Lord, no. Methane’s far too valuable an organic material to burn — even if the laws permitted it. We use it for complex hydrocarbon synthesis.”

“Have you been finding any?”

“More than you would believe.”

It occurred to me that I had an explanation to offer Hermann Jaynsie for Earth’s lack of interest in the food supply contracts. There would never be a shortage of nitrogen on Earth, with an atmosphere that was nearly eighty percent that gas. If they now had enough hydrocarbons, and enough energy, elemental food synthesis would be a snap.

The most surprising thing was Van Lyle’s willingness to tell all this to me, an outsider. Didn’t Earth’s Food and Energy Council care any more who knew what? Or were there missing pieces that were not being mentioned?

“I understand the Geotron,” I said. “But what was that about being underwater?”

“Well, you don’t think we’d put it on land, do you? Solid surface is too precious. We put it on the seabed.” And then, when I looked puzzled. “Captain Roker, just how big do you think the Geotron is?”

“I’ve no idea.”

“Then I’ll tell you. The main ring is forty kilometers across.”

Forty kilometers. A long day’s walk. Or in this case, a long day’s swim.

“So it was built on the Malvinas’ continental shelf,” he went on. “Where there’s a lot of available seabed, and the water is only fifty to a hundred meters deep.”

“Malvinas?”

“Off the east coast of Patagonia. We’ll be there in half an hour. Then you can see it for yourself.”

In the next few minutes I learned that the Malvinas’ coastal zone was now Earth’s hottest development area, site not only of the Geotron but also of the world’s most modern food facilities and genetic laboratories; all, naturally, off-shore, in the shallow seas that ran for hundreds of kilometers east of the mainland.

And while Lyle talked, I struggled to remember where Patagonia was. Southern hemisphere? Definitely. South America? Probably. It occurred to me that although I could quote the size and approximate orbital parameters for every major body from Mercury to the edge of the Oort cloud, I did not know the geography of Earth.

We were flying near the edge of the atmosphere. I stared up at the familiar black sky, with the brightest stars showing, then turned my eyes down to wisps of white cloud, with far below them the alien sea.

I felt, as usual when I was on Earth, a long way from home.

Our descent to the Geotron did nothing to ease my feeling of alienation. I had not realized that our aircar was amphibious, until we were skimming a few feet above long, rolling waves. We touched down, planing across the surface in a cloud of spray. Lyle took his hands off the controls.

And instead of bobbing on the rollers, we kept descending. After a few moments of panic, while the water level rose past the windows and plunged us into a green gloom, I realized that the aircraft was not only an amphibian, it was also a submersible. I could hear the thrum of engines aft, and see the yellow beams of light that lit the way ahead and behind us for many meters.

“Lights for passenger viewing only,” explained Lyle. “Just so you can enjoy the sights. I haven’t been controlling the craft since we touched down on the waves. We’ll be homed in to the Geotron facility automatically — sonic control, of course, not radio. Radio signals won’t travel through water.”

“How far is it?”

“Just a couple of kilometers. There was no point in landing too far away, but the final approach is interesting.”

We had been angling steadily downward. The natural sunlight was vanishing, breaking to cloudy patches of darkening green. Shoals of silver-green fish and what looked like endless thousands of purple squids darted through the beams of our headlights. Then they too were gone, and I had my first sight of Earth’s sea-floor, a smooth grey-brown carpet of fine sediments that swirled up like an ominous mist behind us in the wake of our propulsion jets.

That alien fog made me uneasy. I was much comforted when the huge silver wall of the outer Geotron ring appeared ahead, and we moved to an underwater docking. That felt quite familiar to me. An inward pressure of seawater replaced the space environment’s outward pressure against vacuum, but the same sort of locks were needed. And once we were inside, we could have been in any controlled one-gravity environment between the Vulcan Nexus and the Hyperion Deep Vault.

Van Lyle led the way through the set of connected chambers that formed the control nexus for the Geotron. Other maintenance areas were spaced all around the rim of the main rings, a couple of kilometers apart. After a final up-and-down ride over the top of the inner ring, we were finally spilled by a moving stairway into a big square room, partitioned off into dozens of small work cubicles.

McAndrew sat in one of them with a man and a woman in their early twenties. His shoes and socks were off and he was staring at a listing that filled the whole of the cubicle wall. He looked well, was obviously unrestrained, and gave every impression of a man totally at home and thoroughly enjoying himself.

“There he is,” said Lyle. “I was wondering, why does he take his shoes and socks off when he’s working?”

“In case he needs to count to more than ten.” But I didn’t know the real answer, any more than Lyle did, and I had known Mac for a long time. At least he wasn’t cracking his toe joints at the moment.