My plan was to get on to the fjord and bury the letter, persuade myself that the current might carry it up to the North Pole one day. That’s what I could tell Luke, anyway. Under the snow, I barely noticed the shoreline, but I felt the change underfoot. Hard sea ice, scoured by the wind. Walking across it was like walking across a desert, so wide and flat it makes you dizzy. Mountains framed the fjord on either side, but straight ahead there was nothing except a shimmering line between sky-blue ice and ice-blue sky. And, at the join, a dark figure, a nomad on the horizon.

What if it’s him? screamed the danger signal in my head. I couldn’t tell who it was, no more detail than a Lowry man, but that didn’t get in the way of a good old-fashioned panic. Whoever pushed Hagger over the crevasse, whoever stole the notebooks, whoever hit me over the head and sabotaged the plane: what if it was him?

I almost ran back. Then I got a grip on myself. I shifted the rifle on my shoulder, angled so it was pointing almost ahead, and carried on.

It seemed to take for ever to reach him. Out there, you lose all sense of distance. He was standing very still, staring across to the far side of the fjord with a pair of binoculars. A neat round hole punctured the ice by his feet. Blood smeared the ice around it.

‘Fishing?’ I asked, pointing at the hole.

Ash turned abruptly. ‘It’s you,’ he said, as if I’d done something wrong. ‘I thought you were blotto.’

‘I’ve woken up.’

‘Hah.’ He turned back to his binoculars. I tried to follow his gaze. All I saw was snow.

‘Is there anything out there?’

‘Bear.’ Manners overtook him; he handed me the binoculars. ‘Just to the right of that big boulder.’

A shiver went through me as I put the binoculars to my eyes, though I still couldn’t see it. All the training, all the warnings and briefings, they’d never sunk in to the point I really believed they were real. Now it was out there, a few hundred yards away.

Ash guided my arm until I was pointing in the right direction. Even with the binoculars, I had to look hard to make out his features: the black nose, the legs with their awkward, lumbering gait.

‘It’s a big one,’ said Ash.

Whether the bear caught my scent, or a movement, or a glint from the binoculars, I don’t know. But he stopped, turned his head and stared straight at me. Another shiver. Suddenly, half a mile didn’t seem nearly far enough.

I was glad I’d remembered to bring a rifle, and said so. Ash shuddered as if I’d stepped on his grave. Having devoted his life to the bears, I suppose the thought of shooting one was abhorrent.

‘My first bear,’ I said.

‘You’re lucky. Don’t get many down here these days.’

I wasn’t surprised. I’d seen the pictures in the Guardian, bear cubs marooned on shrinking ice floes waiting to drown.

‘I wonder if there’ll be any left at all by the time my son grows up.’

‘That’s the paradox,’ said Ash. ‘Sea ice is melting faster than ever, earth’s boiling like a kettle, but here on Utgard the bears are thriving.’

‘I thought you said you don’t see them much any more.’

‘We don’t see them here — because they’ve all gone north. The seal population up in the north-west has exploded in the last couple of years. I haven’t seen them this healthy in twenty years. And where the seals go, the bears follow.’

‘Why would that be?’ I wondered aloud.

Ash shrugged. ‘My theory? There’s a current that comes down the west coast. I think that’s warming, so everything in the food chain, from krill to seals, is thriving. Eventually, it’ll kill them if they can’t adapt. But for the moment, they’re in clover.’

I thought of the micro-organisms teeming in the samples from Echo Bay. I thought of the neat row of red-ringed X’s flowing along the west coast on my map, until they stopped in Echo Bay.

‘We’re on the west coast,’ I pointed out. ‘Shouldn’t the current bring them here, too?’

With the toe of his boot, Ash scraped a rough egg shape in the snow. ‘That’s Utgard.’ He made a mark in the bottom, and another halfway up the left-hand side. ‘Zodiac. Echo Bay.’ Digging in his heel, he drew a line that started just above the top of the egg, ran along the left side as far as Echo Bay, then spun away at a right angle.

‘The Stokke current. It—’ He broke off as he realised I was laughing. I couldn’t help it. ‘What?’

‘Stokke’s a make of pram.’ I remembered the yummy mummies at the baby groups when Luke was small, swapping notes on their state-of-the-art baby kit. Space-age designs that looked nothing like Luke’s third-hand relic. I’d hung around on the fringes, the only father there, like the shy boy at a school disco.

‘This Stokke was a polar explorer. Anyway, the current brings cold water down from the far north. But at Echo Bay, it meets the very tail end of the Gulf Stream coming up from the south and gets deflected out west, towards Greenland. That’s why it doesn’t reach here.’

I stared at the diagram he’d drawn. It was crude, but I’d spent so long looking at the map in Hagger’s lab I could visualise it easily.

‘So this current goes past the Helbreensfjord.’

‘That’s right. In summer, ice from the Helbreen calves off and floats down to Echo Bay. Played havoc with the rig there last year, I heard.’

He took the binoculars back off me and scanned the horizon.

‘He’s gone. Let’s go in.’

We trudged back over the flat, frozen fjord. Across the ice, Zodiac looked a long way away, a Matchbox model dwarfed by the mountains.

Two Englishmen, even in a frozen wilderness at the end of the earth, will always end up talking about the weather.

‘Nice day,’ said Ash. And it was. White snow, blue sky and pure light crystallising everything.

‘Hard to believe Eastman and Kennedy are trapped in Vitangelsk by the wind,’ I said.

Ash gave me a look. ‘I didn’t know they’d gone up there.’

He sounded unhappy about it.

‘They radioed in. Apparently, they found a bear.’

I couldn’t see his face, between the hood and the beard and the icicles hanging off his eyebrows, but he seemed to tighten up at the news.

‘A bear? At Vitangelsk?’

‘I thought you’d be interested.’

We carried on, two lone figures on a crystal plain. I glanced back one more time, in case the bear had decided to follow us, but of course I couldn’t see him.

When I got back to the Platform, I realised I’d forgotten to post Luke’s letter.

Woken at 4 a.m. by footsteps in the corridor. I lay in bed, wishing there were locks on the doors. Wishing I’d borrowed one of the rifles from the rack. Being trapped at Zodiac with someone who might want to kill me is bad enough. The fact that he’s got full access to a well-stocked gun cabinet at the end of the hall terrifies me.

The footsteps passed my room and headed towards the front door. Towards the gun rack. I heard the boot-room door squeak open — but not shut. He was trying not to wake anybody. Maybe he didn’t want to make himself unpopular.

I slipped out of my bunk and poked my head round the door. Moving suddenly, in case anyone was there.

The corridor was empty, the boot-room door shut.

I walked to the end of the corridor. Zodiac at 4 a.m.’s a ghostly place: full daylight leaching through the windows, but not a soul to be seen. As if the aliens came and abducted everyone except you. The only sound was a soft wind sighing through the aerials on the roof. It seemed to have risen since yesterday evening.

Just as I got to the boot room, the outer door closed with an unmistakable thud, and the click of the latch. I heard footsteps descending the metal stairs outside.

I checked the rack on the wall. All the guns were there.