"But surely the chances — "

"What are the chances of mid-air collision to the only two iiruaft occupying ten thousand square miles of sky? On paper, they don't exist. There have been three such collisions this year already. So we keep the sonar pinging. But the really important eye, when you're under the ice, is the one that looks up. Come and have a look at it."

He led the way to the after-starboard end of the control room, where Dr. Benson and another man were busy studying a glassed-in eye-level machine that outwardly consisted of a seven-inch-wide moving ribbon of paper and an inked stylus that was tracing a narrow, straight black line along it. Benson was engrossed in adjusting some of the calibrated controls.

"The surface fathometer," Swanson said. "Better known as the ice machine. It's not really Dr. Benson's machine at all, we have two trained operators aboard, but as we see no way of separating him from it without actually courtmartialing him, we take the easy way out and let him be." Benson grinned, but his eye didn't leave the line traced out by the stylus. "Same principle as the echo-sounding machine: it just bounces an echo back from the ice — when there is any. That thin black line you see means open water above. When we move under the ice, the stylus has an added vertical motion which not only indicates the presence of ice but also gives us its thickness."

"Ingenious," I said.

"It's more than that. Under the ice it can be life or death for the «Dolphin». It certainly means life or death for Drift Station Zebra. If we ever get its position, we can't reach it until we break through the ice, and this is the only machine that can tell us where the ice is thinnest."

"No open water at this time of year? No leads?"

"'Polynyas,' we call them. None. Mind you, the ice pack is never static, not even in winter, and surface-pressure changes can very occasionally tear the ice apart and expose open water. With air temperatures such as you get in winter, you can guess how long the open water stays in a liquid condition. There's a skin of ice on it in five minutes, as inch in an hour, and a foot inside two days. If we ge to one of those frozen-over polynyas inside, say, three days we've got a fair chance of breaking through."

"With the conning tower?"

"That's it. The sail. All new nuclear subs have specially strengthened sails designed for one purpose only: breaking through Arctic ice. Even so we have to go pretty gently as the shock, of course, is transmitted to the pressure hull."

I thought about this for a moment and then said, "What happens to the pressure hull if you come up too fast — as I understand may happen with a sudden change in salinity and sea temperature — and you find out at the last minute that you've drifted away from the indicated area of thin ice and have ten solid feet of the stuff above you?"

"That's it," he said. "Like you say, it's the last minute. Don't even think about such things, much less talk about them: I can't afford to have nightmares on this job." I looked at him closely, but he wasn't smiling any more. He lowered his voice. "I don't honestly think that there is one member of the crew of the «Dolphin» who doesn't get a little bit scared when we move in under the ice. I know I do. I think this is the finest ship in the world, Dr. Carpenter, but there are still a hundred things that can go wrong with it, and if anything happens to the reactor or the steam turbines or the electrical generators, then we're already in our coffin and the lid screwed down. The ice pack above is the coffin lid. In the open sea, most of those things don't matter a damn: we just surface or go to snorkel depth and proceed on our diesels. But for diesels you need air — and there's no air under the ice pack. So if anything hap. pens we either find a polynya to surface in, one chance in ten thousand at this time of year, before our standby battery gives Out, or… well, that's it."

"This is all very encouraging," I said.

"Isn't it?" He smiled, none too soon for me. "It'll never happen. What's the worthy Benson making all the racket about?"

"Here it is," Benson called. "The first drift block. And another. And another! Come and have a look, Doctor."

I had a look. The stylus, making a faint, soft hissing sound, was no longer tracing out a continuously horizontal line, but was moving rapidly up and down across the paper, tracing out the outline of the block of ice passing astern above us. Another thin, straight line, more agitated vertical movements of the stylus, and again another block of ice had gone. Even as I watched, the number of thin, horizontal lines became fewer and fewer and shorter and shorter until eventually they disappeared altogether.

"That's it, then," Swanson nodded. "We'll take her deep now, real deep, and open up all the stops."

When Commander Swanson had said he was going to hurry, he'd meant every word of it. In the early hours of the following morning I was awakened from a deep sleep by a heavy hand on my shoulder. I opened my eyes, blinked against the glare of the overhead light, then saw Lieutenant Hansen.

"Sorry about the beauty sleep, Doc," he said cheerfully. "But this is it."

"This is what?" I said irritably.

"85° 35' north, 21 °20' east — the last estimated position of Drift Station Zebra. At least, the last estimated position with estimated correction for polar drift."

"Already?" I glanced at my watch, not believing it. "We're there already?"

"We have not," Hansen said modestly, "been idling. The skipper suggests you come along and watch us at work."

"I'll be right with you." When and if the «Dolphin» managed to break through the ice and began to try her one-in-amillion chance of contacting Drift Station Zebra, I wanted to be there.

We left Hansen's cabin and had almost reached the control room when I lurched, staggered, and would have fallen but for a quick grab at a hand rail that ran along one side of the passageway. I hung on grimly as the «Dolphin» banked violently sideways and around like a fighter plane in a tight turn. In my experience, no submarine had ever been able to begin to behave even remotely in that fashion. I understood now the reasons for the safety belts on the diving-control seats.

"What the hell's up?" I said to Hansen. "Avoiding some underwater obstruction ahead?"

"Must be a possible polynya. Some place where the ice is thin, anyway. As soon as we spot a possible like that, we come around like a dog chasing its own tail, just so we don't miss it. It makes us very popular with the crew, especially when they're drinking coffee or soup."

We went into the control room. Commander Swanson, flanked by the navigator and another man, was bent over the plotting table, examining something intently. Farther aft, a man at the surface fathometer was reading out ice-thickness figures in a quiet, unemotional voice. Commander Swanson looked up from the chart.

"Morning, Doctor. John, I think we may have something here."

Hansen crossed to the plot and peered at it. There didn't seem to be much to peer at — a tiny pinpoint of light shining through the glass top of the plot and a squared sheet of chart paper marked by a most unseamanlike series of wavering black lines traced out by a man with a pencil following the track of the tiny moving light. There were three red crosses superimposed on the paper, two very close together, and just as Hansen was examining the paper the crewman manning the ice machine — Dr. Benson's enthusiasm for his toy did not, it appeared, extend to the middle of the night — called out "'Marl!" Immediately the black pencil was exchanged for a red and a fourth cross made.

"'Think' and 'may' are just about right, Captain," Hansen said. "It looks awfully narrow to me."

"It looks the same way to me, too," Swanson, admitted. "But it's the first break in the heavy ice that we've had in an hour, almost. And the further north we go, the poorer our chances. Let's give it a try. Speed?"