'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.