Other jars seemed to contain not pigments but medicinal herbs – black and white hellebore ground to a powder, poisonous but having many uses; the holosteon or 'all-bone' plant (perversely named by the Greeks because it is entirely soft, just as they call gall 'sweet') with its slender, hairlike roots, good for closing wounds and healing sprains; white lathyris seeds, good for curing dropsy and drawing away bile. I was just replacing the lid on a tiny jar full of aconitum, also called panther's-death, when someone cleared his throat behind me. The door slave watched me disapprovingly from the hallway.

'You should be careful before you stick your nose in the jars,' he said. 'Some of the things inside can be very poisonous.'

'Yes,' I agreed, 'like this stuff. Aconitum – they say it sprang from the mouth foam of Cerberus when Hercules pulled him up from the Underworld. That's why it grows near openings to the Underworld, like the Jaws of Hades. Good for killing panthers, I'm told… or people. I wonder why your mistress keeps it.'

'Scorpion stings,' the slave answered curtly. 'You mix it with wine to make a poultice.'

'Ah, your mistress must be very wise about such things.'

The slave crossed his arms and stared at me like a basilisk. I slowly replaced the jar on the shelf and left the room.

I decided to take a walk along the cliffs beyond the village. The afternoon sun was warm, the sky was crystal. A progression of clouds scudded along the blue horizon, and overhead gulls circled and shrieked. The fog that had blanketed the coast an hour before had vanished. The Sibyl of Cumae began to seem unreal, like the vapours that rose from Lake Avernus, as if all that had happened since we left Baiae that morning were a waking dream. I breathed deeply of the sea air and was suddenly weary of the villa in Baiae and its mysteries. I longed to be in Rome again, walking through the crowded streets of the Subura, watching the gangs of boys who play trigon in the squares. I longed for the secluded quiet of my own garden, the comfort of my own bed, and the smell of Bethesda's cooking.

Then I saw Olympias climbing up a narrow trail from the beach. In one hand she carried a small basket. She was still quite distant, but I could see that she was smiling – not the ambiguous smile that she wore in Gelina's villa, but a true smile, radiant and content. I also saw that the hem of her short riding stola was dark, as if she had been wading in water up to her knees.

I looked beyond her and tried to imagine where she had come from. The trail she was taking vanished from sight among a tumble of rocks, and I could see no beach at all at the water's edge. If she wanted to gather shells or sea creatures, there must surely be better and safer places in the vicinity of Cumae.

As she drew nearer I hid behind a stone. I circled behind it, trying to find a way to watch her without being seen, and noticed a movement from the corner of my eye. A hundred paces away I saw what might have been my mirror image, had I been wearing a dark hooded cloak and worn a long pointed beard. The philosopher Dionysius stood just as I did, poised behind a rock on the edge of the cliff, furtively watching Olympias climb up the hillside.

He did not see me. I moved slowly around the stone, concealing myself from Olympias and Dionysius both, and then scurried away from the cliff until I was out of sight. I hurried back to Iaia's house and rejoined Eco on the terrace.

Olympias arrived a few moments later. The door slave spoke to her in a hushed tone. Olympias stepped, into another room. When she reappeared some moments later, she had changed into a dry stola and no longer carried her basket.

'Was your visit to the Sibyl fruitful?' she asked, smiling pleasantly.

Eco frowned and averted his eyes. 'Perhaps,' I said. 'We'll find out on the way back to Baiae.'

Olympias looked puzzled, but nothing could dampen her buoyant mood. She walked about the terrace, caressing the flowers that bloomed in their pots. 'Shall we go back soon?' she asked.

'I think so. Eco and I still have work to do, and Gelina's house will no doubt be in much confusion, such as always occurs on the day before a great funeral.'

'Ah, yes, the funeral,' Olympias whispered gravely. She nodded thoughtfully, and the smile almost faded from her lovely Lips as she bowed her head to smell the flowers.

'The sea air agrees with you,' I said. She looked more beautiful than ever, with her eyes shining brighdy and her golden hair swept back by the wind. 'Did you take a walk along the beach?'

'A short walk, yes,' she said, averting her eyes.

'When you came in the door a moment ago, I thought I saw you carrying a basket. Gathering sea urchins?'

'No.'

'Shells?'

She looked uneasy. 'Actually, I didn't go to the beach.' The sparkle in her eyes became opaque. 'I walked along the ridge instead. I gathered some pretty stones, if you must know. Iaia uses them to decorate the garden.'

'I see.'

We left shortly thereafter. As we walked through the foyer towards the door, I saw that Olympias had not bothered to conceal her basket when she entered but had left it in plain sight in the corner opposite the door slave's stool. While Olympias stepped through the door into the sunlight, I lingered behind. I stepped towards the basket and lifted the cover with my foot. There were no stones within. Except for a small knife and a few crusts of bread, the basket was empty.

The passage through the stone maze and across the bald, windy hills seemed quite different in the bright sunshine, but when we began to enter the woods around Lake Avernus I sensed the same atmosphere of uncanny seclusion that I had felt before. I looked back occasionally, but if Dionysius followed he kept himself out of sight.

It was not until we came to the precipice that I told Olympias I wanted to stop. 'But I showed you the view already,' she protested. 'You can't want to see it again. Think what a beautiful day it must be down in Baiae.'

'But I do want to see it,' I insisted. While Eco found a place to tether the horses, I located the beginning of the path on the left side of the slab, just as the Sibyl had described. The opening was obscured by overgrown brush and old branches, and the path itself was faint and disused. There was no sign of fresh footsteps in the fog-dampened earth, not even the mark of a deer. I pushed through the brush with Eco behind me. Olympias protested but followed.

The path descended in sharp switchbacks over barren, rocky ground. The odour of sulphur grew ever stronger, borne on a wave of hot, rising air, until we were compelled to cover our faces with our sleeves. At last we found ourselves on a wide, shallow beach of yellow mud. The lake was not a uniform liquid surface, as it appeared from above, but a series of interconnected pools of sulphur overhung by clouds of vapour and separated by bridges of rock that might have been used to traverse to the other side, if a man cared to take the risk and could survive the heat and the smell. The stench of the bubbling pits was almost overpowering, but I thought I detected an even more unwelcome odour borne on the reek.

I looked up. We stood almost directly below the shelf of rock from which we had descended. In the face of the cliff I could see no cave or any other sign of shelter. I shook my head, more dubious than ever of the Sibyl's word.

'How can anyone possibly meet us here?' I grumbled to Eco. 'I'd sooner expect to see the Minotaur come strolling up this beach than one of Gelina's escaped slaves.' Eco gazed up and down the beach, as far as the obscuring mists allowed. Then he raised his eyebrows and pointed at something at the water's edge only a few feet away.

I had seen the thing already and had taken no notice of it, thinking it was only a piece of driftwood or some natural detritus thrown up by the lake. Now I looked at it more closely, and realized with a shock what it must be.