There was a long silence, followed by a rhythmic squeaking noise that was almost certainly one of the bolts being turned. Agata restrained herself from cheering; Ramiro was asleep.

It took Tarquinia more than a chime to loosen all six bolts and remove the access panel. Agata peered over her shoulder into the exposed cavity, where cooling pipes ran along the back of the rebounders. If one of the banks of rebounders had failed, someone could have squeezed in here to fit a replacement.

‘Anything?’ Agata asked hopefully.

‘Nothing obvious,’ Tarquinia admitted. ‘I thought this was the last place we hadn’t poked around in, but maybe I should sit down with the maintenance logs to confirm that.’

‘Right.’

Tarquinia lingered, lowering her head partway through the hatch and turning her face sideways. ‘There’s a big stone beam that goes right across the top of the engines, from rim to rim.’

‘Could something be attached to it?’ Agata suggested. ‘Out of sight from where you are?’

‘I’m just wondering why it’s there at all,’ Tarquinia replied. ‘The floors of the cabins should provide enough bracing for the engines. And why a beam that runs across one particular diameter of the disc, and not another one at right angles to it? Nothing about the stress from the engines picks out one axis like that.’

‘No.’

Tarquinia said, ‘If I don’t come out in six lapses, send in Azelio with a rope.’

‘Azelio?’

‘No offence to you or Ramiro, but he’s the skinniest. There’s not much point in two people getting stuck.’ Tarquinia climbed head first through the access hatch, slithering deeper and humming softly as the cooling pipes banged against her, until even her feet had disappeared from view.

Agata waited, listening intently for any cries of discovery or distress. She was starting to wonder if she should have kept her inspiration to herself. Tarquinia trapped in the guts of the Surveyor would not be a happy outcome – and if she actually located the mythical device there could be worse to follow.

Worrying silences were punctuated with thuds, pings and echoing curses. Finally, Agata heard Tarquinia returning, her steady advance eliciting a resonant hum from the maze of pipes.

‘That was exhausting,’ she said. ‘Can you give me a hand up?’

Agata jumped down into the shaft and helped her out through the hatch. The flesh of Tarquinia’s torso had become corrugated as she’d forced it between the pipes, giving her the appearance of a decoratively shaped novelty loaf on legs.

‘Any luck?’

Tarquinia said, ‘There’s nothing hidden beside the beam.’

‘Oh.’

‘But the beam itself is hollow.’

‘Really? How can you tell?’

‘You can hear it when you tap,’ Tarquinia explained.

‘Couldn’t that just be to save mass?’