'Stop her mouth!' Tiaan cried.

One of the slaves wound a strip of cloth three times around Tirior's head and pulled it tight. Tiaan felt the flow ease. Her heart was beating irregularly and she felt faint. So close.

'You taught me the value of your word, Tirior.' Tiaan wrenched open the pack, Tirior had been reaching for a small glass tube, capped in gold, with a scintillating powder inside. Tiaan tossed it into the tar and pressed it down with one of the walker's feet. 'Bind them, please. Merryl.'

Cord was found in a storeroom and the three prisoners' hands bound behind their backs.

'I'm not your enemy, Tiaan,' said Nish. I was wrong about you before. I'm sorry.'

He seemed different to the Nish Tiaan had known. He was more sure of himself, less angry, and made no attempt to fight those who held him. But Tiaan could not forgive so easily. 'Every time I've met you I've regretted it, Nish,' she said wearily.

'We were looking for you, to bring you out of here.'

Tiaan activated the walker and moved away. 'I have a plan,' she whispered to Merryl.

'I thought you must.'

'I think, with my crystal, that I may be able to operate the construct. If you can direct me to the way out, it will carry us through the fumes. For a while, at least.'

'I know every tunnel/ he said.

'Lift me into the construct and I'll see what I can do. The tar around it will have to be cleared away.'

'I'll have it done.'

Taking the amplimet from the walker, Tiaan put it in her pocket, undid the straps and lifted herself on her arms. Merryl carried her across. 'I'm not too heavy, am I?'

He smiled. 'You're no burden at all.'

He boosted her up the side and she slid her legs in. As her feet struck the floor Tiaan's knees buckled. Her muscles might have been made from cloth. Pulling out the operator's seat she sat down hurriedly.

The layout was much the same as in her thapter. She pressed the small recessed button and a hexagonal tube sprang out. Flipping the cap open, she removed the crystal, which was pale blue and striated down the sides. She had never seen one like it. Slipping it into her pocket, she put the amplimet in its place. In her own construct, or thapter as she had called it after learning how to make it fly, she'd made a special device to reduce power.

Tlaan hoped that would not he necessary here, since she was drawing from such a distant node. In any case, she had nothing to build it with.

She pressed the hexagonal tube in and closed the cap. After a long moment, a faint whine came from below, and a subtle tremor. It was working!

It took hours to remove the great gouts of sticky tar, and the work was so exhausting that the slaves had to rest after every few strokes. The job had just been completed when Merryl cried, Tiaan, look out!'

She got the hatch down just in time, as an even bigger clot buried the construct completely. By the time that had been removed, the air inside was stale. A day had gone by since her escape from the patterner.

The black miasma, which had advanced and retreated a number of times, was now flowing steadily across the floor. It would be up to their knees within minutes.

'Better bring the prisoners on board,' she said to Merryl, who was anxiously watching the fumes. Tiaan popped the amplimet out and pocketed it, just in case. There was no room for trust; the whole world seemed to be against her.

The prisoners were brought in and taken below. Minis gazed sorrowfully at her, like a dog that had been kicked. Nish, who looked as though he hadn't slept in days, simply lay down, pillowed his head on his arms and went to sleep. Tirior showed no expression at all. She was the one to watch.

Everyone came aboard save the two who were mattocking away at the sticky tar on the right-hand side. When the black fog was at the level of their thighs, Tiaan called them in. Should a sudden surge overwhelm the construct now, it would be impossible to get out.

Merryl set guards on the Aachim and Nish. The remaining slaves went below, leaving just her and Merryl in the operator's compartment. It would be very cramped down there, with nine passengers. Tiaan reinserted the amplimet and took hold of the trumpet-shaped lever. The whine rose in pitch but the construct did not move.

'It's still stuck in the tar,' said Merryl. ' I don't think -'

'I'll try to work it free.'

He peered anxiously ahead. A billow of black mist was rolling towards them. Tiaan pulled down the hatch and fastened it. It became dark inside, except for the subtle glow from the plate in front of her. The front panel thinned to transparency. The outside was dimly lit by glowing globes that shone intermittently through the fog.

She wiggled the lever back and forth, ever so gently. The whine rose and fell. With a delicate shudder the construct pulled free and rose in the air until its base was at the level of the black fog. Tiaan edged it forwards.

'Straight ahead or to the left?' she said, after they'd been travelling a while.

'The way out into the main pit is straight ahead, but we may not be able to get through that way . . .' Merryl was looking at her expectantly. 'Is something the matter?'

She realised that she was frowning. I originally came here looking for Gilhaelith. He's a strange, unlikeable fellow, but he was good to me.' Even though he'd cared more for the amplimet than about her safety, Tiaan had to know that he was safe.

'He's an important man,' said Merryl. 'Surely the lyrinx will have taken him with them.'

'I was important to them, yet they panicked and left me behind. They may have abandoned him as well. Do you know where Gilhaelith was working?'

'In a tunnel excavated into the Great Seep.'

A tunnel in liquid tar? How can that be?'

'They froze it first.'

'How?' said Tiaan curiously.

'One of their Arts.'

'If he was left behind, can he possibly still be alive?' she said to herself.

'Not if he's still in the seep.' He looked through the front. 'But, perhaps, in the tunnels near it ... We can go that way. It's not much further.'

Merryl was a man of the same heart as Tiaan. She thanked him, silently. 'He treated me kindly. I have to know.'

'Then go straight on.'

They came to a high point in the tunnel where the heavy black mist had not reached. Merryl cracked the hatch open to let in fresh air, but it stank so badly that he quickly closed it again. The construct went down sharply, plunging into fumes which the globes could not penetrate. Tiaan had to creep along, and even then was continually bumping into the sticky, gritty walls.

They turned a sharp bend, then another that formed the other half of an 'S', and the black fog thinned. Ahead, two tunnels diverged at a shallow angle.

'Which way?' said Tiaan.

Merryl was staring blankly through the screen. 'I'm . . , not sure. The fog has confused me. Have we missed an intersection?'

'We could have missed fifty for all I could see.'

'Take the left. I think: After a few minutes, Tiaan felt the right-hand side scrape on the sandy wall. Shortly afterwards the other side did the same and the construct shuddered to a stop.

'It's the wrong way, said Merryl. 'Better go back.'

'I hope we can,' Tiaan muttered.

After much jerking and heaving the construct began to move backwards. They had been heading down the other tunnel for some minutes when Tiaan saw a red glow in a cross-tunnel to their left.

'We're running out of options,' said Merryl. 'Can you go faster?'

She increased speed as much as she dared, following a zigzag path away from the burning area until they hit a broad tunnel that ran straight. There were no fumes in it and they made good time. The walls and roof here were yellow sandstone, hardly tar stained at all. After ten minutes they came abruptly into blackened rock and then, where the tunnel opened out, into solid tar. The tunnel kept on.

'Is this where Gilhaelith was?' Tiaan did not like the feel of the place.

'He would have been some way ahead. We're close to the outer edge of the Great Seep - the solid edge. In a few spans it becomes soft and beyond that it's liquid tar for a league.'

'How did they tunnel it? And why?'

Merryl spoke to the huddled slaves in a language Tiaan did not know. A woman answered in the same tongue.

'They used devices powered by phynadrs,' said Merryl, 'to draw the heat out and freeze the tar hard. Why, I cannot say, only that it was mighty important to them. Matriarch Gyrull worked there every day, and a matriarch does not risk her life needlessly.'

They crept on. Objects were strewn here and there as if discarded in flight - rotting, tar-stained remnants of clothing, a small wooden chest. Further on was a distinctly human-looking body.

Tiaan caught her breath. Not Gilhaelith, surely? She drew the construct alongside, opened the hatch and looked down.

The body was small, female, and tar-impregnated. 'It has a .., withered look; Tiaan said. 'As if long dead.'

'Many people, and many animals, must have become stuck in the tar over the aeons, and been carried down into the depths. I saw a number of them over my time here, all perfectly preserved. You need shed no tear for her, Tiaan. She's been dead hundreds of years, at the very least.'

'I'll go on, just in case . . .' She edged the construct down the tunnel. I thought you said they tunnelled in a long way.'

About a hundred spans, I heard.'

'We're only in twenty and I can see the end,' said Tiaan.

She lifted herself up on the side, the better to see. The end of the tunnel was but spans away, a smooth, shining black bulge dotted with fragments of wood and cloth. 'It's moving!' Warm tar was creeping towards them like molasses squeezed through a hole. The tunnel had collapsed, 'If Gilhaelith was in there, he's dead.'

Five.

Merryl gripped her shoulder. 'Was he special to you, Tiaan?' 'I wouldn't say that we were friends, for he had none. Gilhaelith was quite the strangest man I've ever met, and totally absorbed in himself. Yet he was good to me and I can't forget it. We'd better go, if we're to get out.'

Reversing the little construct, Tiaan turned it about and went back the way they had come. At the first intersection, Merryl said, 'Go left.'

She headed that way but was soon confronted by a baleful glow and another creeping fume.

'There's fire ahead, Tiaan. Try the other way.'

To the right they encountered a cave-in that completely blocked the tunnel. There was no hope of clearing it, for the fumes were knee high and rising. They turned back to the junction and took the middle path, their last hope.

'Fire,' Merryl said dully, after they had moved less than a hundred spans.

Tiaan kept going until it was certain there was no way past. 'What now?'

'Resign ourselves to death.'

It was hot here. Tiaan went back to the entrance to the tar tunnel. She could not resign herself to dying. Turning the construct again, she stared at the oozing face of the tar.

'Tell me about the Great Seep, Merryl.'

'It's a good league across, and hundreds of spans deep. Some say it's bottomless. Things, and creatures trapped in it, sink down and sometimes appear again, countless years later, with the wheeling of the slow currents in its depths.'

'If we remain here,' she said absently. 'We'll be dead within the hour.'

'I'd-say so.'

'How long would the air in the construct last with the hatch down, and all of us inside?'

'I don't know. Two hours? Three? Four, possibly.'

'Then let's live those extra hours. Let's risk it.' Tiaan slammed the hatch, took a deep breath and moved the construct gently forwards until it met the convex face of the tar.

Merryl's eyes met hers. Tiaan's eyes were alive for the first time since he'd met her. 'What have we got to lose?'

The construct met resistance and stalled. Tiaan moved the controls, just a tickle. The skinned tar broke and the machine surged into treacly material that smeared across the screen. Everything went black.

'Are we even moving?' whispered Tiaan. 'I can't tell.'

Merryl looked through the rear screen. 'We're going about two spans a minute. The tar's coming over the top. I can't see anything now.'

She nudged the trumpet-shaped lever. There was no sense of motion. 'It's not fast enough. It'll take an hour to get to the end and we've still got to go up to the top of the seep. How far below ground are we?'

He shrugged. 'More than a hundred spans, but less than two hundred.'

'That's another hour, probably two. Can we make it before we breathe all our air?'

'I don't know.'

'I'll have to go faster.'

'Go too fast and it may tear the construct apart.'

'Too slowly and it won't matter' she retorted.

The minutes ticked by. Occasionally they came up against an object that scraped along the skin of the construct. It was hot inside now.