The Heads’ huge helmet-skulls were sensitive to heat — fantastically so, enabling the Heads to track and kill with almost perfect accuracy. Heads were deadly opponents, Cilia-of-Gold reflected. But the people had nowhere else to go.

“We’ve come a long way, to reach this place, Strong-Flukes. If we had to undergo another journey—” through more cold, stagnant tunnels “ — many of us couldn’t survive. And those who did would be too weakened to fight.

“No. We have to stay here — to fight here.”

Strong-Flukes groaned, wrapping her carapace close around her. “Then we’ll all be killed.”

Cilia-of-Gold tried to ignore the heavy presence of the Seeker within her — and its prompting, growing more insistent now, that she get away from all this, from the crowding presence of people — and she forced herself to think.

Larionova followed Kevan Scholes up the slope of the wall-mountain. Silicate surface dust compressed under her boots, like fine sand. The climbing was easy — it was no more than a steep walk, really — but she stumbled frequently, clumsy in this reduced gee.

They reached the crest of the mountain. It wasn’t a sharp summit: more a wide, smooth platform, fractured to dust by Mercury’s wild temperature range.

“Chao Meng-Fu Crater,” Scholes said. “A hundred miles wide, stretching right across Mercury’s South Pole.”

The crater was so large that even from this height its full breadth was hidden by the tight curve of the planet. The wall-mountain was one of a series that swept across the landscape from left to right, like a row of eroded teeth, separated by broad, rubble-strewn valleys. On the far side of the summit, the flanks of the wall-mountain swept down to the plain of the crater, a full mile below.

Mercury’s angry Sun was hidden beyond the curve of the world, but its corona extended delicate, structured tendrils above the far horizon.

The plain itself was immersed in darkness. But by the milky, diffuse light of the corona, Larionova could see a peak at the center of the plain, shouldering its way above the horizon. There was a spark of light at the base of the central peak, incongruously bright in the crater’s shadows: that must be the Thoth team’s camp.

“This reminds me of the Moon,” she said.

Scholes considered this. “Forgive me, Dr. Larionova. Have you been down to Mercury before?”

“No,” she said, his easy, informed arrogance grating on her. “I’m here to oversee the construction of Thoth, not to sightsee.”

“Well, there’s obviously a superficial similarity. After the formation of the main System objects five billion years ago, all the inner planets suffered bombardment by residual planetesimals. That’s when Mercury took its biggest strike: the one which created the Caloris feature. But after that, Mercury was massive enough to retain a molten core — unlike the Moon. Later planetesimal strikes punched holes in the crust, so there were lava outflows that drowned some of the older cratering.

“Thus, on Mercury, you have a mixture of terrains. There’s the most ancient landscape, heavily cratered, and the planitia: smooth lava plains, punctured by small, young craters.

“Later, as the core cooled, the surface actually shrunk inwards. The planet lost a mile or so of radius.”

Like a dried-out tomato. “So the surface is wrinkled.”

“Yes. There are rupes and dorsa: ridges and lobate scarps, cliffs a couple of miles tall and extending for hundreds of miles. Great climbing country. And in some places there are gas vents, chimneys of residual thermal activity.” He turned to her, corona light misty in his face plate. “So Mercury isn’t really so much like the Moon at all… Look. You can see Thoth.”

She looked up, following his pointing arm. There, just above the far horizon, was a small blue star.

She had her face plate magnify the image. The star exploded into a compact sculpture of electric blue threads, surrounded by firefly lights: the Thoth construction site.

Thoth was a habitat to be placed in orbit close to Sol. Irina Larionova was the consulting engineer contracted by Superet to oversee the construction of the habitat.

At Thoth, a Solar-interior probe would be constructed. The probe would be one Interface of a wormhole, loaded with sensors. The Interface would be dropped into the Sun. The other Interface would remain in orbit, at the center of the habitat.

Thoth’s purpose was to find out what was wrong with the Sun.

Irina Larionova wasn’t much interested in the purpose of Thoth, or any of Superet’s semi-mystical philosophizing. It was the work that was important, for her: and the engineering problems posed by Thoth were fascinating.

The electric-blue bars she could see now were struts of exotic matter, which would eventually frame the wormhole termini. The sparks of light moving around the struts were GUTships and short-haul flitters. She stared at the image, wishing she could get back to some real work.

Irina Larionova had had no intention of visiting Mercury herself. Mercury was a detail, for Thoth. Why would anyone come to Mercury, unless they had to? Mercury was a piece of junk, a desolate ball of iron and rock too close to the Sun to be interesting, or remotely habitable. The two Thoth exploratory teams had come here only to exploit: to see if it was possible to dig raw materials out of Mercury’s shallow — and close-at-hand — gravity well, for use in the construction of the habitat. The teams had landed at the South Pole, where traces of water-ice had been detected, and at the Caloris Basin, the huge equatorial crater where — it was hoped — that ancient impact might have brought iron-rich compounds to the surface.

The flitters from Thoth actually comprised the largest expedition ever to land on Mercury.

But, within days of landing, both investigative teams had reported anomalies.

Larionova tapped at her suit’s sleeve-controls. After a couple of minutes an image of Dolores Wu appeared in one corner of Larionova’s face plate. Hi, Irina, she said, her voice buzzing like an insect in Larionova’s helmet’s enclosed space.

Dolores Wu was the leader of the Thoth exploratory team in Caloris. Wu was Mars-born, with small features and hair grayed despite AntiSenescence treatments. She looked weary.

“How’s Caloris?” Larionova asked.

Well, we don’t have much to report yet. We decided to start with a detailed gravimetric survey…

“And?”

We found the impact object. We think. It’s as massive as we thought, but much — much — too small, Irina. It’s barely a mile across, way too dense to be a planetesimal fragment.

“A black hole?”

No. Not dense enough for that.

“Then what?”

Wu looked exasperated. We don’t know yet, Irina. We don’t have any answers. I’ll keep you informed.

Wu closed off the link.

Standing on the corona-lit wall of Chao Meng-Fu Crater, Larionova asked Kevan Scholes about Caloris.

“Caloris is big,” he said. “Luna has no impact feature on the scale of Caloris. And Luna has nothing like the Weird Country in the other hemisphere…”

“The what?”

A huge planetesimal — or something — had struck the equator of Mercury, five billion years ago, Scholes said. The Caloris Basin — an immense, ridged crater system — formed around the primary impact site. Whatever caused the impact was still buried in the planet, somewhere under the crust, dense and massive; the object was a gravitational anomaly which had helped lock Mercury’s rotation into synchronization with its orbit.

“Away from Caloris itself, shock waves spread around the planet’s young crust,” Scholes said. “The waves focused at Caloris’ antipode — the point on the equator diametrically opposite Caloris itself. And the land there was shattered, into a jumble of bizarre hill and valley formations. The Weird Country… hey. Dr. Larionova.”