«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?»