'And was anything found?'

'For several hours, almost nothing. Nothing that might have rewarded the men for their dangerous work in the shark-infested waters. There was no sign of the log and portolan charts, documents for which the Navy Office would have paid a handsome sum. By noon there was little left of the galleon but her keel, and what the fire had spared the wind and waves dispersed. Sir Ambrose was about to order his men to return-a Spanish frigate had been spotted along the coast-but then a party of them raised something from the shallows. It was scorched and waterlogged but still intact.'

'Yes?' I was holding my breath. 'What was it?'

'A sea-chest,' she replied. 'But not just any sea-chest, for it was made of the same wood as the ship. Carved on one of the sides was the coat of arms of a man named Pinzón.'

'The captain,' I said eagerly.

She shook her head. 'Francisco Pinzón was the navigator, and a famous one at that, a graduate of the School of Navigation and Cartography in Seville. He had been the pilot of the Quirós expedition in search of the Solomon Islands in 1606. He must have thrown the chest overboard with all else, but it survived both the fire and the wreck, because sandalwood is as durable as it is beautiful. Once opened, it was found to be filled with books, for the distinguished Señor Pinzón was apparently an avid reader. Most were stories of knightly endeavour, but there was another book inside the chest besides these tales of chivalry, one that told its own tale of a dangerous and impossible quest.'

'The copy of Ortelius.'

'Yes. The Prague edition of the Theatrum orbis terrarum, a book so rare that in those days not even Sir Ambrose had seen a copy. He had just opened the cover when suddenly one of the salvors rushed into the cabin. Something else had been found in the water.'

It was another clue: dozens of scraps of paper from a log or journal that someone had attempted to shred before throwing overboard. The pieces were painstakingly collected from the water, then Sir Ambrose dried the scraps and carefully reassembled them on the desk in his cabin. The task took the better part of the afternoon and was made difficult because many of the scraps were missing or else illegible. At first he could take his bearings from only a few words: TOLEDO, LONGITUDO, IUPITER. By this time the Spanish frigate was scarcely a league away, and a larger fleet had been sighted off the coast of Hispaniola. But the Philip Sidney would not be caught. She weighed anchor and soon after nightfall had reached the islands of the Bahamas. And so it was there among the palmed cays, in dark waters infested with both sharks and pirates, that Sir Ambrose finished assembling what remained of the scraps and, with them, the secret of the Sacra Familia.

'Was it another map?' I asked.

'No,' she replied. 'Something much more intricate than a map. Perhaps you wish to see it?' She had risen to her feet. 'What remains is still quite legible.'

I too found my feet, but the motion seemed to unsettle me and I felt dizzy once again. I wavered on my feet as I followed her across the tiles into the atrium, which was filled with an eerie storm light. The rain on the windows seemed louder now, and the chandelier was chiming noisily overhead. Water had begun trickling down the marble staircase, dripping from the banisters and puddling on the floor, but Alethea was either oblivious or apathetic, for she guided me past the little waterfall, tugging gently at my arm and saying something about an almanac. Her voice was half muffled by the rain. The floor seemed to tremble underfoot as we picked our way along the corridor, passing the Great Room and the breakfast parlour. Suddenly the crypt plunged abysmally beneath us.

'… transits, eclipses, occultations,' her voice was echoing against the copper-sheathed walls as we descended into the entombed air. As we reached the bottom of the stairs I felt water beneath my feet. It seemed to be flowing down the walls, for when I brushed against one of them my shoulder came away wet to the touch. Oily-looking waves streamed past us. Alethea was walking more swiftly now, splashing along in her buskins, still apparently oblivious to the conditions.

'Everything in the tables has been calculated with the utmost precision.' Her voice seemed distant as she strode into the darkness ahead of me, hoisting aloft the creaking lantern. From all round came the sounds of invisible water lisping and hissing as it coursed swiftly along the rocky striations. 'The almanac was compiled, you see, by Galileo himself.'

So it was that I found myself back inside the muniment room, the place where I first encountered, through his many fragments, the mysterious Sir Ambrose Plessington. I lingered on the threshold. The floor, like that in the corridor, was running with water. The sodden rushes squelched as Alethea picked her way across to the coffin, which still sat on the trestle-table, safe for the time being. As she hooked the lamp to the wall sconce I was surprised to see how the water was almost crimson in colour. A droplet of what looked like blood fell from the ceiling and spattered my knuckles.

'Venetian red,' she explained. 'I've been using it in my search for the underground waterways. I pour dye into the cress-pond in order to determine what course it takes. I suppose I might have used a colour that was less gruesome, but as it happens the dye has done its work and I've managed to track down a number of the hidden channels. An engineer is laying pipes and building drains so that the springs can be tamed and the water diverted for use in fountains.'

I wiped my hand on my doublet and stood in silence as she creaked open the lid of the coffin and began to rummage among the papers. I could hear the dull roar of the water as it carved its mysterious channel behind the stone. Tame such waters? I had to admire her optimism, the unfailing buoyancy of her dreams. Even in the midst of such wreckage she could still cling to her grandiose visions of the house. But I had to admire her, I supposed, in other ways as well. For I had come to Pontifex Hall in anger and hatred but now found, almost to my chagrin, that it was impossible to dislike her. Perhaps I was as deluded as she was; perhaps I too was dreaming and desiring even as I trod the rising waters.

'Here it is.'

Her voice startled me from my reverie. She had turned round and was extending in her hand a piece of paper, or some other backing, on to which dozens of scraps had been pasted. Yet another text, another scrap to tell the story of her father's life. As she angled it into the light of the lamp I could see three or four columns of figures, each broken by an occasional gap.

'The puzzle of the Sacra Familia,' she was saying, 'fitted together by Sir Ambrose. Can you read it? The tables predict the eclipses of each one of the Jovian satellites.'

I blinked hard at this piece of handiwork, still perplexed. 'The Jovian satellites? But I fail to understand what they have to do with-'

And then suddenly I did. The print jumped into focus and seemed to detach itself from the page. The paper was spattered with Venetian red, but I had just made out the words IUPITER and LONGITUDO, when a stone burst from the wall like the stopper from the bung-hole of a cask, followed by a reddish tide.

I stumbled backwards a step, feeling the ice-cold water seep through my boots. Another stone broke free and even more water spilled inside, unfurling like tumbling bolts of russet to curl round our feet. The cataclysm had begun. For a paralysing moment I imagined the entire wall buckling and the pair of us crushed to death beneath tons of water and shattered masonry. Then I splashed forward and snatched Alethea's hand.