The good news: the left tunnel was straight with no side exits, twists ou turns. The bad news: it was a dead end. After sprinting a hundred metres, we ran into an enormous boulder that completely blocked our path. Behind us, the sounds of dragging footsteps and heavy breathing echoed down the corridor. Something – definitely not human – was on our tail.

'Tyson,' I said, 'can toi –'

'Yes!' He slammed his shoulder against the rock so hard the whole tunnel shook. Dust trickled from the stone ceiling.

'Hurry!' Grover said. 'Don't bring the roof down, but hurry!'

The boulder finally gave way with a horrible grinding noise. Tyson pushed it into a small room, and we dashed through behind it.

'Close the entrance!' Annabeth said.

We all got on the other side of the boulder and pushed.

Whatever was chasing us wailed in frustration as we heaved the rock back into place and sealed the corridor.

'We trapped it,' I said.

'Or trapped ourselves,' Grover said.

I turned. We were in a six-metre-square cement room, and the opposite mur was covered with metal bars. We'd tunnelled straight into a cell.

'What in Hades?' Annabeth tugged on the bars. They didn't budge. Through the bars we could see rows of cells in a ring around a dark courtyard – at least three stories of metal doors and metal catwalks.

'A prison,' I said. 'Maybe Tyson can break –'

'Shh,' a dit Grover. 'Listen.'

Somewhere above us, deep sobbing echoed through the building. There was another sound, too – a raspy voice muttering something that I couldn't make out. The words were strange, like rocks in a tumbler.

'What's that language?' I whispered.

Tyson's eye widened. 'Can't be.'

'What?' I asked.

He grabbed two bars on our cell door and bent them wide enough for even a Cyclops to slip through.

'Wait!' Grover called.

But Tyson wasn't about to wait. We ran after him. The prison was dark, only a few dim fluorescent lights flickering above.

'I know this place,' Annabeth told me. 'This is Alcatraz.'

'You mean that island near San Francisco?'

She nodded. 'My school took a field trip here. It's like a museum.'

It didn't seem possible that we could've popped out of the Labyrinth on the other side of the country, but Annabeth had been living in San Francisco all year, keeping an eye on Mount Tamalpais just across the bay. She probably knew what she was talking about.

'Freeze,' Grover warned.

But Tyson kept going. Grover grabbed his arm and pulled him back with all his strength. 'Stop, Tyson!' he whispered. 'Can't toi see it?'

I looked where he was pointing, and my stomach did a somersault. On the second-floor balcony, across the courtyard, was a monster plus horrible than anything I'd ever seen before...