Harry nodded. 'So I understand,' he said.

Very well. Now, even if the theory is wrong and they're not two sides of the same coin, there remains one factor common to both.

'Which is?'

That they're both one-way systems.' Once you enter a black hole, you don't get out again. And once you've been expelled from a white hole, there's no way back in. The way I see it, the same thing applies to your grey holes: this Gate at Perchorsk, and the second Gate which you believe lies somewhere along the course of this underground river.

'One-way systems?'

Each of them! Emphasis on 'each'. You go in through one, and you come out through the other!

It stopped Harry dead in his tracks. Finally he said: That's brilliant! Once you use a Gate, it's out of bounds. Having passed you through, it won't accept you again, no matter which end you started out from. But a second gate will! So all I have to do is find the second Gate. In fact, I already know where it is! It's the Gate the Wamphyri have been using to send their monstrosities through to Perchorsk.'

Ah, but that's what it is, not where it is, said Mobius.

'It's a step in the right direction, anyway,' Harry replied. Then he sobered a little. There is however one small drawback. If I come through that Gate into Perchorsk, they'll not only shoot me, they'll probably fry me, too!'

Ah-! but here Mobius could only shrug.

Thanks, anyway,' said Harry. 'You've confirmed what I was already suspecting: that there have to be two Gates. The Wamphyri have been using one for thousands of years, and now they've started using the new one, which Luchov and his crowd inadvertently blasted into being at Perchorsk. It's the only explanation. So ... if you'll excuse me now I'll be on my way. I have to say goodbye to my mother. She'd never forgive me if I did something like this without telling her.' He sighed. 'She'll want to try to talk me out of it - even knowing she can't. But... she's like that.'

All mothers are like that, Harry, said Mobius, very seriously. Good luck, my boy.

But in fact, luck would have very little to do with it...

The next morning Darcy Clarke met Harry at E-Branch HQ in London, and while Harry checked his equipment, making sure he knew how to use it, Clarke took the opportunity to pass on a little information.

'About this subterranean tributary of the Danube,' he said. 'Harry, it's a death-trap! I had it checked out overnight. Our man in Bucharest looked into it for you. The place is known well enough and we have its exact location. There are newspaper clippings concerning it, various bits of documentation. The locals have an immemorial dislike for it. In 1966 a couple of cavers took it on. It was summer and the tributary was running dry at the time. Four hours after they started in there was a flash flood up in the mountains. One body was washed out, the other lost forever. And these were experts.'

'And they were walking and swimming,' said Harry. 'I won't be.'

'Eh?'

'I said I was going up to the watercourse, but I didn't say how.'

Clarke gasped, 'You'll use the Mobius Continuum?'

'Of course.'

'Then why the wet-suit, aqualung and all?'

'Just in case.'

Clarke fell silent for a moment, then said: 'I was only trying to help.'

'And I appreciate it,' Harry told him. 'But I know my own business best.'

Ten minutes later he took up his gear in a waterproof kit-bag and went to Radujevac. From the outskirts of the town he caught a taxi into the countryside close to the location Clarke had given him. He paid his driver with money from the same source. With his bag over his shoulder he set off down a country track, eventually arriving at his destination. It was a wild place and there was no one around. Harry dumped his kit in a copse and covered the bag with dead branches, then returned to the site of the resurgence.

It was in the base of a cliff, overgrown with ivy, where limestone outcrops glistened with moisture. To the north-east stood the grey, forbidding Carpathians, and to the south sloping wooded countryside. Harry stood on the banks of the pool under the cliffs and looked again at the mountains, then down at the dark water where it gurgled into view from untold caverns of gloom. It issued from the mouth of a cave. This was the spot where Old Belos of the Wamphyri first entered our world. And others like him. And between here and those legendary mountains, somewhere underground, lay a gateway to an alien world. Now it was Harry's task to pin-point that Gate as accurately as possible before setting out up the river to find it.

He checked again that there was no one around, no one to see him take his departure. The place was silent, wooded, where only the birds sang and the cold water gurgled. But this time Harry was well muffled-up and didn't feel the cold.

He picked a spot in the foothills to the north-east, went there via the Mobius Continuum. The door from which he exited was the same as always: a 'hole' in the universe, with nothing to distinguish it from hundreds of other doors Harry had used. Harry moved again, even closer to the looming peaks. But this time when he emerged, the 'edges' of his door shimmered a little. It was the warning he'd been looking for, which told him he was close.

Very close, Harry, said a dead voice in his mind, surprising him. It felt like someone had come up close behind him and whispered in his ear.

'Do I know you?' He scanned the countryside around,' looked down on distant towns, Radujevac, Cujmir, Recea. They were smoky, wintry smudges on his horizon.

No, but I know you. Your mother has been making enquiries on your behalf.

Harry sighed. 'She means well,' he said. 'Has she disturbed you?'

Not at all. I'm happy to help. You intend to travel the length of the Radujevac resurgence, right? The voice was full of excitement, eagerness. Which was what gave its owner away.

'You were a caver,' said Harry. 'You died back in the summer of 1966, somewhere up that underground river.'

That's me, said the other, a little sadly. And I never did get to finish the job. My name is Gari Nadiscu, and if I'd made it the bore would have been named after me: the Nadiscu Route. It was a dream. Maybe you can finish it for me?

Harry said: 'Wait,' and transferred to the Mobius Continuum. 'Now talk to me,' he said. 'I want to get closer to you.' He followed the other's thoughts, emerged at the very foot of the mountains. And again the Mobius door shimmered, more than before, confirming Harry's belief that he was moving closer to the gate.

'You didn't do badly,' he told Nadiscu. 'You covered, oh, maybe nine miles before that flood hit you! Are you still down there?' He glanced at the stony mountain soil under his feet. 'I mean, is there anything left... you know? How did you get trapped? Your companion was washed out.'

Trapped, answered the other, grimly. That's the right word, Harry. I crawled onto a ledge. There was a crack in the wall. As the water rose I climbed deeper into the crack. Finally I got jammed, couldn't move. I was wearing a lung, of course. It was a bad time. I lasted as long as my air...

That must have been pretty terrible,' Harry commiserated. But:

Don't waste time on that, said the other. You've things to do. How can I help you?

Two things,' said Harry. 'One: what was the course of the river up to the time you... when the flood came? And two: how deep are you, as you calculate it, under the surface?'

Nadiscu supplied the answers and Harry thanked him. 'I won't be looking for the river's source,' he admitted, 'for it's a different kind of source that interests me. But if it all works out I'll come back some time and tell you how far I got. OK?'

Thanks, Harry. I'd appreciate that.

Harry used the Continuum and moved on into the mountains, exiting on a steep, pine-covered slope. This time the interference was such that Harry knew he was almost there. Directly below him, at some great depth in the roots of the mountains, the Gate to the world of the Wamphyri was waiting for him.