‘A short cut,’ Elena had mumbled, but then lost her bearings at the bottom of the steps when they got to a tubular room lined with pipes, where you had to duck to avoid hitting your head.

Martin felt as if he’d been transported inside a submarine of the sort he’d seen in films. The pipes had vents that could be opened with bilious-green wheel valves. There was a cupboard that seemed to consist of several fuse boxes with numerous instruments whose needles were barely moving.

Elena responded to his question as to which direction they should take by calling the captain on her phone and asking him the way. Martin was amazed that Bonhoeffer was able to understand his fiancée given that she was still mumbling, but after a brief exchange he appeared to have put her on the right track because she pointed to the left and let him go first.

The route brought them to a white bulkhead which required considerable strength to open. Martin needed both hands to shift the wheel and push open the metal door that was as thick as that of a large safe.

The room beyond was wider and darker. It smelled of dust and diesel. Dirt littered the floor and spiders’ webs hung from the instruments inside the cupboard, which looked older than the ones they’d just passed.

‘Where are we?’ he asked Elena, who was leaning exhausted against one of the dusty boxes.

‘No idea. An old control room. Over there…’ she said, gesturing to another door on the far side, too weak to finish the sentence.

Martin went in the direction she’d indicated.

He stepped on thoughtlessly discarded screws, tissues, paper and other rubbish that hadn’t been disposed of for ages, and at the end of the room came to another bulkhead, which was even more difficult to open than the previous one.

Beyond it he discovered a cathedral.

Or at least that was his first impression when he crossed the threshold into a room the height of a house, lit merely by a strip of halogen lamps on either wall.

At the far end – in the altar area, so to speak – was a shimmering oval copper tube, not unlike a brewing copper. Two thirds of its shell bulged into the room, while the rear third was integrated into the Sultan’s outer hull. A sort of fire ladder ran up the vessel and vanished into the darkness five metres above Martin’s head.

‘Here it is!’ he called out, to let the doctor know that he’d found the blue shelf. As they’d descended into the depths of the ship’s hull she’d explained to him where the term came from and why the waste disposal plant was no longer in use.

He looked back, but couldn’t see Elena in the entrance, nor did she answer him.