«The positioning must be significant,» Wai datavised. «They built it almost at the centre of the wreck. They wanted to give it as much protection as possible.»

«Agreed,» Katherine replied. «They obviously considered it important. After a ship has suffered this much damage, you don't expend resources on anything other than critical survival requirements.»

«Whatever it is,» Schutz reported, «it's using up an awful lot of power.» He was walking round it, keeping a respectful distance, wiping a sensor block over the floor as he went. «There's a power cable feeding each of those legs.»

«Is it radiating in any spectrum?» Marcus asked.

«Only that light you can see, which spills over into ultraviolet, too. Apart from that, it's inert. But the energy must be going somewhere.»

«OK.» Marcus walked up to a buttress, and switched his collar focus to scan the aperture. It was veiled by a grey haze, as if a sheet of fog had solidified across it. When he took another tentative step forward the fluid in his semicircular canals was suddenly affected by a very strange tidal force. His foot began to slip forwards and upwards. He threw himself backwards, and almost stumbled. Jorge and Karl just caught him in time.

«There's no artificial gravity underneath it,» he datavised. «But there's some kind of gravity field wrapped around it.» He paused. «No, that's not right. It pushed me.»

«Pushed?» Katherine hurried to his side. «Are you sure?»

«Yes.»

«My God.»

«What? Do you know what it is?»

«Possibly. Schutz, hang on to my arm, please.»

The cosmonik came forward and took her left arm. Katherine edged forward until she was almost under the lambent doughnut. She stretched up her right arm, holding out a sensor block, and tried to press it against the doughnut. It was as if she was trying to make two identical magnetic poles touch. The block couldn't get to within twenty centimetres of the surface, it kept slithering and sliding through the air. She held it as steady as she could, and datavised it to run an analysis of the doughnut's molecular structure.

The results made her back away.

«So?» Marcus asked.

«I'm not entirely sure it's even solid in any reference frame we understand. That surface could just be a boundary effect. There's no spectroscopic data at all, the sensor couldn't even detect an atomic structure in there, let alone valency bonds.»

«You mean it's a ring of energy?»

«Don't hold me to it, but I think that thing could be some kind of exotic matter.»

«Exotic in what sense, exactly?» Jorge asked.

«It has a negative energy density. And before you ask, that doesn't mean anti-gravity. Exotic matter only has one known use, to keep a wormhole open.»

«Jesus, that's a wormhole portal?» Marcus asked.

«It must be.»

«Any way of telling where it leads?»

«I can't give you an exact stellar coordinate; but I know where the other end has to emerge. The xenocs never called for a rescue ship, Marcus. They threaded a wormhole with exotic matter to stop it collapsing, and escaped down it. That is the entrance to a tunnel which leads right back to their homeworld.»

•   •   •

Schutz found Marcus in the passenger lounge in capsule C. He was floating centimetres above one of the flatchairs, with the lights down low.

The cosmonik touched his heels to a stikpad on the decking beside the lower hatch. «You really don't like being wrong, do you?»

«No, but I'm not sulking about it, either.» Marcus moulded a jaded grin. «I still think I'm right about the dish, but I don't know how the hell to prove it.»

«The wormhole portal is rather conclusive evidence.»

«Very tactful. It doesn't solve anything, actually. If they could open a wormhole straight back home, why did they build the dish? Like Katherine said, if you have an accident of that magnitude then you devote yourself completely to survival. Either they called for help, or they went home through the wormhole. They wouldn't do both.»

«Possibly it wasn't their dish, they were just here to investigate it.»

«Two ancient unknown xenoc races with FTL starship technology is pushing credibility. It also takes us back to the original problem: if the dish isn't a distress beacon, then what the hell was it built for?»

«I'm sure there will be an answer at some time.»

«I know, we're only a commercial trader's crew, with a very limited research capability. But we can still ask fundamental questions, like why have they kept the wormhole open for thirteen thousand years?»

«Because that's the way their technology works. They probably wouldn't consider it odd.»

«I'm not saying it shouldn't work for that long, I'm asking why their homeworld would bother maintaining a link to a chunk of derelict wreckage?»

«That is harder for logic to explain. The answer must lie in their psychology.»

«That's too much like a cop-out; you can't cry alien at everything you don't understand. But it does bring us to my final query. If you can open a wormhole with such accuracy across God knows how many light-years, why would you need a starship in the first place? What sort of psychology accounts for that?»

«All right, Marcus, you got me. Why?»

«I haven't got a clue. I've been reviewing all the file texts we have on wormholes, trying to find a solution which pulls all this together. And I can't do it. It's a complete paradox.»

«There's only one thing left, then, isn't there?»

Marcus turned to look at the hulking figure of the cosmonik. «What?»

«Go down the wormhole and ask them.»

«Yeah, maybe I will. Somebody has to go eventually. What does our dear Katherine have to say on that subject? Can we go inside it in our SII suits?»

«She's rigging up some sensors that she can shove through the interface. That grey sheet isn't a physical barrier. She's already pushed a length of conduit tubing through. It's some kind of pressure membrane, apparently, stops the ship's atmosphere from flooding into the wormhole.»

«Another billion-fuseodollar gadget. Jesus, this is getting too big for us, we're going to have to prioritize.» He datavised the flight computer, and issued a general order for everyone to assemble in capsule A's main lounge.

•   •   •

Karl was the last to arrive. The young systems engineer looked exhausted. He frowned when he caught sight of Marcus.

«I thought you were over in the xenoc ship.»

«No.»

«But you . . .» He rubbed his fingers against his temples. «Skip it.»

«Any progress?» Marcus asked.

«A little. From what I can make out, the molecular synthesizer and its governing circuitry are combined within the same crystal lattice. To give you a biological analogy, it's as though a muscle is also a brain.»

«Don't follow that one through too far,» Roman called.

Karl didn't even smile. He took a chocolate sac from the dispenser, and sucked on the nipple.

«Katherine?» Marcus said.

«I've managed to place a visual-spectrum sensor in the wormhole. There's not much light in there, only what soaks through the pressure membrane. From what we can see it's a straight tunnel. I assume the xenocs cut off the artificial gravity under the portal so they could egress it easily. What I'd like to do next is dismount a laser radar from the MSV and use that.»

«If the wormhole's threaded with exotic matter, will you get a return from it?»

«Probably not. But we should get a return from whatever is at the other end.»

«What's the point?»

Three of them began to talk at once, Katherine loudest of all. Marcus held his hand up for silence. «Listen, everybody, according to Confederation law if the appointed commander or designated controlling mechanism of a spaceship or free-flying space structure discontinues that control for one year and a day then any ownership title becomes null and void. Legally, this xenoc ship is an abandoned structure which we are entitled to file a salvage claim on.»