I looked at the empty bottle. I’d thought they were solutes for the machines, but now I read the label I saw it was something else. Rhodamine B hydrological dye. Caution: Stains.

There was only one person at Zodiac I knew who used that in her research — and Hagger was supposed to have broken it off with her months ago. No wonder Greta was furious.

You’d think, with two dozen people confined to a few hundred square metres of ice, it would be easy to find anyone you wanted at Zodiac. It took me the best part of an hour to track Annabel down; eventually, I found her on a pair of skis gliding around the perimeter. I wondered if we’d been chasing each other round in circles all morning, like Pooh and Piglet.

‘I’ve been in the bang shop.’ She waved her pole at one of the older, wooden huts, where they keep the explosives. ‘Have you seen anyone go in there? Twenty kilos of det cord’s gone missing.’

‘Don’t look at me.’

‘No,’ she agreed, in a way that seemed to imply I wouldn’t be up to it. ‘Probably one of the American students who doesn’t understand the metric system.’

She pushed off and began to ski away. I followed.

‘I want to ask you about Martin.’

She didn’t slow down. I took the Rhodamine-B bottle out of my pocket and threw it in front of her. ‘What was he doing with that?’

‘You’ve just violated article nine of the Utgard Treaty,’ she told me. ‘Littering.’ She stopped, bent down and picked it up.

‘I found it in his lab. Half a dozen of them. I just want to know what you were doing there.’

‘He must have taken them from my store. I haven’t been in his lab in six months.’

‘Greta thinks you have.’

Wrong thing to say. She started skiing again; I had to run to keep up.

‘What was Martin doing with that dye? Did he come to you for help?’ I was starting to sweat. Lithe and long-legged, Annabel seemed to glide effortlessly over the snow. ‘You use Rhodamine to trace water in the ice, right? Was Hagger interested in something coming off the glacier in meltwater?’

She stopped and looked back. She wasn’t even breathing hard. ‘Be careful. Last time you and I went running around the ice together, you fell down a hole.’

‘Did Martin ask you about the glacier? Please,’ I added. A cramp was spreading through my side. I knelt down in the snow and pressed my hand on my knee. I probably looked ridiculous.

Annabel surveyed me, the Ice Queen looking down on her subject.

‘Hagger asked me about the ionic profile of some water samples. That’s all.’

‘What did you find?’

‘High sulphite levels.’

I struggled. Sulphite’s a mineral, nothing to do with enzymes and proteins. Nothing really to do with biology at all.

But Annabel was holding something back. Waiting for me, behind her mirrored glasses.

‘What do high sulphite levels mean?’

‘Glaciers don’t just push ice down to the sea. They’re complex hydrological systems with their own chemistry. Ice is an insulator; it sits on top of the rock like a heavy blanket. Now, the rock has heat in it from the earth’s core, and that heat can’t escape, so it melts the bottom of the glacier. The whole thing’s sitting on a water slide. As water flows through the channels, or over the rock, it gathers its own chemical signature. Chloride means it’s come from melted snow; calcite means it’s travelled under the glacier, in contact with the rock.’

‘And sulphite?’

‘Sulphite doesn’t show up in meltwater much.’

‘But you said—’

‘There are only a few places in the world where it happens, so we don’t have much data. But where there are mine workings that go underneath a glacier — and, as I said, that’s not common — we’ve seen evidence that the mines can become part of the glacier’s drainage system. Meltwater seeps into the tunnels, and then joins the glacier again and flows out at the bottom.’

‘And sulphite ions are evidence of that.’

She nodded. ‘Digging tunnels exposes the rock to air. The air oxidises the metals in the rock, which produces sulphite. Then water flows through and washes it out.’

I’ve got enough chemistry that I could follow that. ‘So Martin showed you some of his samples.’

‘Yes.’

‘And the sample bags — do you remember if they had coloured dots on them? Red or green?’

‘All the ones that tested positive for sulphite had red dots—’

‘You’re sure?’

In my eagerness, I’d jumped in too quickly and cut her off. She shot me a dirty look.

‘I asked him about the dots. I thought he might be playing a joke on me. But he said all the red ones came from the Helbreensfjord, which makes sense. All the mines around Vitangelsk — the tunnels go on for kilometres. Some must go right through the mountain and under the Helbreen.’

‘So Hagger’s samples from the Helbreensfjord — the red ones — contained water that had gone through these mine tunnels under the glacier and come out at the bottom.’

‘The data’s consistent with that hypothesis.’

‘And Rhodamine B would prove it. I mean, if you poured some of that dye in at the top of the Helbreen, and found it coming out at the bottom full of sulphites, that would be the proof.’

‘I’ve never put dye around there.’

‘But Hagger asked you to.’

‘It’s not in my project. And unlike some people, I do what my funding body are paying me to do. That’s why they keep funding me.’

I waved the dye bottle. ‘So he tried it himself.’

She sniffed. ‘Even if he did, he couldn’t have interpreted the data. Finding Rhodamine in the outlet water, after it’s travelled ten kilometres underground in who knows which direction, isn’t an easy thing. Hagger couldn’t even pour it straight. He had the dye all over his hands when he died.’

She pushed back a slender leg, then shot it forward to begin skiing away.

‘Still does, unless anyone’s cleaned him up.’

The cold store at Zodiac feels like a morgue at the best of times. Long racks filled with ice cores, steel boxes spiked with frost. The body at the end, wrapped in plastic sheeting, almost seemed natural. Almost.

I unwrapped the plastic and examined Hagger’s body. It didn’t bother me as much as I’d thought: the flesh was frozen so hard, I couldn’t think of it as ever having been alive. As Annabel had said, a pinkish dye stained the hands, like a child who’d been overenthusiastic with the felt tips.

Was Hagger really so inept he spilled it all over himself?

I thought I heard a noise behind me and looked round. Nothing there, except the endless rows of ice cores.

The cores sample every snowfall that’s ever happened on this glacier, one on top of the other. You can read them like tree rings.

Hagger had an ice core in his freezer, I remembered. I’d wondered, when I saw it, why a man who studied sea ice would care about the lifeless heart of a glacier. I’d also wondered how he came by it.

The whole thing’s sitting on a water slide.

What if it wasn’t what’s in the ice that interested him?

I ran back to the Platform. Ignoring the party in the mess, I dived into Hagger’s lab and opened the fridge. I’d forgotten the freezer compartment when I’d cleared out the samples, but it was still there: a stubby cylinder of ice that looked as if it had been chopped with an axe.

I took it back to the caboose and put it on the hot plate. Usually, you use the hot plate to break down DNA, but it worked pretty well as a stove. In a few minutes, the core was sinking into a puddle of its own meltwater. I poured it off into a beaker, prepared a solution and ran it through the mass spectrometer.

The graph matched the others perfectly. A bit weaker, but that was understandable: the contaminated water wasn’t part of the glacier, but had flowed under it. The trace must be from residue that had stuck to the bottom.