Then, with a snap, as she could see through the transparent roof, the cables from the winch disengaged. Yet the car continued to descend.

‘This is a pretty fast ride.’

Trant grunted. ‘We had to custom-design the system. We’re going too deep for conventional cables, even under the low gravity. We fitted the car with crawler attachments; we’re clambering down the walls of the shaft.’

‘Too deep,’ Stef repeated. ‘How deep?’

‘Over four hundred kilometres,’ Trant said, with a touch of pride – justifiable, Stef thought. ‘We’re going all the way to the base of the planet’s crust. To the fringe of the mantle, in fact, which is where the kernels are found.’

‘It’s quite a trip,’ King said. ‘All this is hush-hush at present, you understand, but I have got a couple of tame journalists documenting all this for the history books. I’m given to understand that a crewed trip to the edge of a planetary mantle has never been achieved elsewhere, not even on Earth.’

Stef thought over what she knew about Mercury. ‘I thought the prevailing theory is that Mercury suffered a tremendous impact, back in the age of planet formation. The whack shattered and stripped away the planet’s upper rocky layers. Right?’

‘That’s one theory,’ Trant said. ‘There’s another that’s doing the rounds here. Informally, I mean. That whatever incredible event created or implanted the kernels on Mercury might have caused a huge convulsion of energy, a convulsion that nearly blew the planet apart.’

‘Wow,’ Stef said, impressed, but she reached for her native scientific caution. ‘Quite a hypothesis. Have you got any way of proving it?’

King said, ‘I rather think that’s why you’re here, Major Kalinski.’

The elevator car slid to a smooth halt, and the shaft walls seemed to lift like a curtain. Stef found herself looking out into a cavern, flat-roofed, cut into the deep rock. This cave, hundreds of kilometres under Mercury’s surface, was brightly lit, and there were more small domes, pressurised facilities, marked with UN and UEI sigils.

The purpose of the cavern was obvious too. Armed troopers in military-specification pressure suits stood in a loose circle around what looked like an unprepossessing patch of floor. Stef saw that scientific equipment of all kinds had been assembled around this bit of floor; lenses and other sensors peered down, and there was an industrial-strength laser mount. Something about the whole set-up, the sheer bizarreness of finding a science base and security cordon hundreds of kilometres deep under Mercury, made Stef’s heart hammer even harder.

‘Time to close up your suits,’ Trant said. ‘We’ll run through another full integrity check before stepping out of the car.’

Stef was glad of the long minutes of routine that followed. Since arriving in Mercury orbit she felt as if she had fallen too quickly into this place, this pit bored into the deepest rocky heart of the solar system; she needed time for her soul to catch up with her body.

At last the car door opened, the air sighed out, and Stef walked out, heading towards the circle of troopers, the enigmatic patch of floor they protected. She was locked inside her suit, listening to the air-circulation fans and her own noisy breathing. The troopers let them pass. And as they neared the very centre, King and Trant too stepped back, allowing Stef to walk forward alone, staring at the floor.

And there, set in the rock of Mercury, buried under hundreds of kilometres of crustal layers for billions of years until dug out by questing human hands and tools, was –

A hatch.

CHAPTER 38

Stef walked around the emplacement, trying to absorb the physical reality of it. Trying to observe rather than analyse, for now, the best strategy when faced with the utterly unexpected.

What did she see?

She saw a panel, a rough square of some seamless, pale grey material – metal, perhaps, or ceramic, or some unknown material altogether. It was maybe ten metres across. And at the centre of the panel was a circle, a fine seam engraved into the plain material, perhaps three metres in diameter. That was all, there was no further marking or indentation.

She turned to face King and Trant, who looked at her expectantly.

‘Well?’ King snapped. ‘You see why we didn’t tell you? You see why you had to look for yourself? And you see why we told Earthshine? It’s hard to think of a more significant development for the future of the human race.’

‘It’s obviously artificial,’ she said. She turned back. ‘Obviously – a hatch.’

Trant grinned. ‘That’s what everybody calls it. A common first reaction. In the internal reports we capitalise it. The Hatch,’ she said heavily. She turned, gesturing at the walls. ‘There are kernels all through this layer. You could pick them out by hand. And in the middle of this rich lode, we found – this.’

‘How did you detect it?’

‘Initially by traces in deep radar pulses, seismic traces. Some very strange echoes.’

Stef knelt now, beside the emplacement. The panel looked about a couple of centimetres thick. ‘Is it safe to touch?’

‘Be my guest,’ Trant said.

Stef set her right hand on the material. She felt nothing. ‘I wish I didn’t have to wear this damn glove.’

‘The material is actually a good deal cooler than the ambient temperature.’

Stef drew her hand back over the edge of the panel, and felt an odd pulling sensation. She tried again, passing her hand back and forth over the edge; it was a kind of tide, a sideways push, like passing a charged iron rod through a magnetic field.

‘We don’t know what it’s made of,’ Trant said. ‘Needless to say. We’ve tried cutting it, with low-level lasers; it just soaks up the heat. There are more destructive tests we could try, but we’ve been reluctant to go that far.’

Stef knew there had always been loose talk about the kernels possibly being artefacts of intelligence. They might or might not be. It was hard to dismiss the Hatch as anything other than an artefact – what natural process could produce an object with this regularity? ‘Do you think it’s in any way associated with the kernels?’

‘Well,’ King said, ‘we found it in the same layer as a rich kernel lode—’

Trant said, ‘It seems coincidental if they aren’t associated. To find two extraordinary things in one location – assuming a link exists is the simplest hypothesis. Occam’s razor, Major Kalinski?’

She ignored the gentle goad. ‘I don’t suppose you’ve tried opening the Hatch.’

Trant said, ‘That seam, whatever it is, is too fine for most of our tools. We could try harder . . . Anyhow, it would be futile.’

‘How so?’

‘Because the Hatch is just a mask. A plate sitting there on levelled-off rock. There’s nothing underneath it. We’ve proved that with sonic and radar probes, and by drilling into the rock under the Hatch.’ She pointed to a couple of small pits.

Stef got to her knees again and examined the Hatch, running her fingers along its thickness. Again she felt that odd sideways push. ‘Have you measured its volume?’

‘I can tell you the calculation.’ Trant pulled a slate from a pouch in her suit leg, and fiddled with it. ‘We’ve got precise measurements of every dimension—’

‘No. That’s a calculation. Length by breadth by height. Have you measured the volume?’

Trant seemed baffled. ‘No. I mean – how?’

Stef stood up. ‘What have you got in the nature of fluids down here? Water, lubricants . . .’

It took a couple of hours to set up the experiment. They rigged up a dome over the Hatch that would hold pressure, and pumped it full of non-reactive nitrogen. Then they poured in lubricant, an inert hydrocarbon borrowed from the elevator assembly, that flooded the emplacement.

It was fiddly work in pressure suits and with improvised equipment, but once Stef had communicated what she wanted the engineers worked quickly and effectively, even though some of them grumbled about the risk of wasting the lubricant, a precious resource here on Mercury. It was always the same with engineers, Stef had observed; nothing made them happier than to be given a well-defined and achievable task, and to be left alone to get on with it.