The sails were reefed and we lay idle in the shallows between Preservation and Rum. Connolly made soundings: three fathoms off the bow and six off the stern. In front of us stood a single exposed boulder, no wider than the beam of the Moonbird. Argyle studied the boulder, squinted west at Rum, north at Preservation and down at his chart. He stabbed a finger emphatically on the paper. ‘This is the place.’

The doctor appeared now, carrying the timber box he had shown me early in our voyage. He requested the boat and it was lowered, then we undertook the painstaking task of getting my father into it. As always, his confidence outstripped his ability, and he attacked the rope ladder with far too much haste. But once we were in, the doctor pushed away from the Moonbird and rowed us a few yards clear of her shadow, then lay down the oars and dropped a small sand anchor. When he was satisfied that it had bitten and the little boat had settled, he lifted the box over the edge of the boat so that its glassed end sat on the surface.

‘Miss Grayling,’ he said, as though we had just met in the street. ‘Press the end into the water and then look in from the top. You may wish to lift that shawl of yours over your head so that the daylight does not enter.’

I did as he instructed, and he moved himself to the other side of the jolly-boat so that my leaning over did not tip us too far. And thus positioned I stuck my head deep into the viewing box. The world inside was green, lit by unseen sources like a night at the theatre. We were suspended over a great timber bowl, and it took me a long time to understand what I was looking at.

In my ears I had the doctor’s voice, telling me that Alexander the Great had once ordered a diving bell made of glass: ‘It carried him beneath the seas and there he beheld a fish so great that it took three days and three nights to pass.’

My eyes were deciphering the wide green world below us. They fell upon a great pile of bottles and split casks and I understood: we were directly over the hull of the Sydney Cove.

I stared in wonder. After long moments I realised my father was asking me what I could see, and I knew that this was a moment of great significance for him. I spoke into the viewing box and heard my own words boom around my ears. I must have sounded strangely muffled to the two men.