"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."