The first part was easy. The trail, sometimes not more than eight or nine feet broad, more often than not opened out into a shelf wide enough, almost, to be called the flat floor of a valley, and we made rapid progress. At noon – I'd warned Hillcrest that we would be traversing the Vindeby Nunataks then and would have to miss our regular radio schedule – we were more than half-way through and had just entered the narrowest and most forbidding defile in the entire crossing when Corazzini came running up alongside the tractor and waved me down to a stop. I suppose he must have been shouting but I'd heard nothing above the steady roar of the engine: and, of course, I'd seen nothing, because they had all been behind me and the width of the tractor cabin made my driving mirror useless.

Trouble, Doc," he said swiftly, just as the engine died. "Someone's gone over the edge. Come on. Quick!"

"Who?" I jumped out of the seat, forgetting all about the gun I habitually carried in the door compartment as an insurance against surprise attack when I was driving. "How did it happen?"

"The German girl." We were running side by side round a corner in the track towards the little knot of people forty yards back, clustering round a spot on the edge of the crevasse. "Slipped, fell, I dunno. Your friend's gone over after her."

"Gone after her!" I knew that crevasse was virtually bottomless. "Good God!"

I pushed Brewster and Levin to one side, peered gingerly over the edge into the blue-green depths below, then drew in my breath sharply. To the right, as I looked, the gleaming walls of the crevasse, their top ten feet glittering with a beaded crystalline substance like icing sugar, and here not more than seven or eight feet apart, stretched down into the illimitable darkness, curving away from one another to form an immense cavern the size of which I couldn't even begin to guess at. To the left, more directly below, at a depth of perhaps twenty feet, the two walls were joined by a snow and ice bridge, maybe fifteen feet long, one of the many that dotted the crevasse through its entire length. Jackstraw was standing on this pressed closely into one edge, holding an obviously dazed Helene in the crook of his right arm.

It wasn't hard to work out Jackstraw's presence there. Normally, he was far too careful a man to venture near a crevasse without a rope, and certainly far too experienced to trust himself to the treachery of a snow-bridge. But, when Helene had stumbled over the edge, she must have fallen heavily – almost certainly in an effort to protect her broken collar-bone – and when she had risen to her feet had been so dazed that Jackstraw, to prevent her staggering over the edge of the snow-bridge to her death, had taken the near-suicidal gamble of jumping after her to stop her. Even in that moment I wondered if I would have had the courage to do the same myself. I didn't think so.

"Are you all right?" I shouted.

"I think my left arm is broken," Jackstraw said conversationally. "Would you please hurry, Dr Mason? This bridge is rotten, and I can feel it going."

His arm broken and the bridge going – and, indeed, I could see chunks of ice and snow falling off from the underside of the arch on which he was standing! The matter-of-fact lack of emotion of his voice was more compelling than the most urgent cry could possibly have been. But for the moment I was in the grip of a blind panic that inhibited all feeling, all thought except the purely destructive. Ropes – but Jackstraw couldn't tie a rope round himself, not with an arm gone, the girl couldn't help herself either, both of them were helpless, somebody would have to go down to them, and go at once. Even as I stared into the crevasse, held in this strange motionless thrall, a large chunk of niv6 broke off from the side of the bridge and plummeted slowly down into the depths, to vanish from sight, perhaps two hundred feet below, long before we heard it strike the floor of the crevasse.

I jumped up and raced towards the tractor sled. How to belay the man who was lowered? With only eight or nine feet between the edge of the crevasse and the cliff behind, not more than three men could get behind a rope, and, with perhaps two men dangling at the end of it what possible purchase could those three find on that ice-hard snow to support them, far less pull them up? They would be pulled over the edge themselves. Spikes – drive a spike into the ground and anchor a rope to that. But heaven only knew how long it would take to drive a spike into the icy surface with no guarantee at the end that the ice wouldn't crack and refuse to hold, and all the time that snow-bridge crumbling under the feet of the two people who were depending on me to save their lives. The tractor, I thought desperately – perhaps the tractor. That would take any weight: but by the time we'd disconnected the tractor sled, pushed it over the edge and slowly backed the tractor along that narrow and treacherous path, it would have been far too late.

I literally stumbled upon the answer – the four big wooden bridging battens sticking out from the end of the tractor sled. God, I must have been crazy not to think of them straight away. I grabbed a coil of nylon rope, hauled out one of the battens -Zagero was already beside me pulling at another – and ran back to the spot as fast as I could. That three-inch thick, eleven-foot long batten must have weighed over a hundred pounds, but such is the supernormal strength given us in moments of desperate need that I brought it sweeping over and had it in position astride the crevasse, directly above Jackstraw and Helene, as quickly and surely as if I had been handling a half-inch plank. Seconds later Zagero had laid the second batten alongside mine. I stripped off fur gloves and mittens, tied a double bowline in the end of the nylon rope, slipped my legs through the two loops, made a quick half-hitch round my waist, shouted for another rope to be brought, moved out and tied my own rope to the middle of the planks, allowing for about twenty feet of slack, and lowered myself down hand over hand until I was standing beside Jackstraw and Helene.

I could feel the snow-bridge shake under my feet even as I touched it, but I'd no time to think about that, it would have been fatal if I had even begun to think about it. Another rope came snaking down over the edge and in seconds I had it tied round Helene's waist so tightly that I could hear her gasp with the pain of it: but this was no time for taking chances. And whoever held the other end of the rope up above was moving even as quickly as I was, for the rope tightened just as I finished tying the knot.

I learned later that Helene owed her life to Mahler's quick thinking. The dog-sledge carrying Marie LeGarde and himself had stopped directly opposite the spot where Helene had gone over, and he had shouted to Brewster and Margaret Ross to sit on it and thread the rope through the slats on the sledge top. It had been a chance, but one that came off: even on that slippery surface their combined weights were more than enough to hold the slightly built Helene.

It was then that I made my mistake – my second mistake of that afternoon, though I did not realise that at the time. To help those above I stooped to boost her up, and as I straightened abruptly the suddenly increased pressure proved too much for the already crumbling bridge. I heard the ominous rumble, felt the snow begin to give under my feet, released my hold on Helene – she was already well clear anyway – grabbed Jackstraw by the arm and jumped for the other side of the bridge a second before the spot where we had been standing vanished with a whroom and went cascading down into the gloomy depths of the crevasse. At the full extent of my rope I hit the ice on the far side of the crevasse, wrapped both arms tightly round Jackstraw – I heard his muffled expression of pain and remembered his injury for the first time – and wondered how long I could hold him when that side of the bridge went too, as go it must, its support on the far side no longer existing. But, miraculously, for the moment it held.