In case I revived Marrin’s nightmare of pre-historians flipping in and out of his river I tackled him very cautiously. Rome seemed a safe subject, too solid, too eternally present for mysteries, and I knew he was well up to date in the long history of the Forest. So over the evening drinks I ventured to ask him if he had ever spotted any underwater foundations at Woolaston which would help archaeologists to decide whether the Romans had built a commercial wharf or just a breakwater to shelter the naval commander’s galley.

His sane and pleasant side at once appeared, or was allowed to appear. It may be that I have not sufficiently emphasised his personal charm, for it stands to reason that without it he could never have kept his commune together and loyal, religion or no religion.

‘Why don’t we both look?’ he replied. ‘It occurs to me that we are of the same build and I have a spare suit. We’ll dive in daylight, of course, and at slack water. That will be about eight to eight-thirty tomorrow, if it isn’t too early for you.’

It was the fourth morning of my stay. After we had grabbed a light breakfast he unlocked the door of a tiled cubicle which led off the guests’ washroom at the end of the passage. There he kept his diving kit. I was glad to see that his own suit and the spare were of Neoprene, for I was used to the wet suit. He had only surface life-jackets, but as we were unlikely to go deeper than twenty feet or so they were good enough and saved trouble.

He had chosen to go in off the Guscar Rocks which I had never seen. The west bank of the Severn is concealed till you reach it. Rough farm tracks lead down to it from the main road to Wales and either stop at the railway embankment or go under it, always past a notice that bathing is extremely dangerous. Beyond the embankment are the flat and empty meadows, without a living thing but the sheep to take an interest in what you do or how you are equipped, abruptly ending at the immensity of the tideway, itself also empty.

It was near the time of low water, and the Guscar Rocks were jagged islands of weed standing above the last eddies of the ebb. It seemed impossible to get at them, for the shore, beneath a miniature cliff of crumbling red rock, was of mud and shale; but Marrin confidently led the way along the banks of a pill until we could swim on the surface across to the rocks, land on them and jump in from the far side clear of the weed.

The water was warmer than I expected and despite its colour of pale milk chocolate – gold if one wants to be politer to Severn than I feel – visibility was not too bad. All I could discover close under the Woolaston shore was that somebody at some time had built something. Not very satisfactory! A narrow and difficult secondary channel led to the old port at Lydney and may have offered a straight run up-river in Roman times. If it did, the Guscar Rocks sheltered a natural harbour. But only dredging could reveal solid evidence, and I doubt if even that expensive process would have much success against the silt and sand brought down by every relentless tide.

We pulled out again on to the rocks, facing the smooth and shining slope of grey mud and the meadows beyond. The green railway embankment, acting as a sea wall, cut us off from the world of the land. Behind us the Severn was still just alive, sucking and swirling as the channel emptied the last of the ebb left behind in the backwaters of the Shepherdine Sands.

The mysterious pattern of the eddies stopped. The Severn was empty and waiting. It was time to swim ashore.

‘The tide is making now,’ Marrin said as we took off masks and aqualungs. ‘We may see the beginning of the bore.’

He had to point it out. There at Woolaston, well before the narrowing of the river, it was more a sudden rise of water level than a wave. Only the surge around the Guscar Rocks revealed the force and speed of the tide which, twenty minutes later up the horseshoe bend with a south-west wind behind it, would have raced on as the wave of the bore, all of five feet high. Meanwhile the Shepherdine Sands, which had seemed a golden playground for children, were rippling water.

When we had crossed the embankment, and our horizon again became the dark tumble of the Forest looking down across the peaceful belt of farm and pasture to its incalculable estuary, I suspected that the face of the Guscar rock from which we had jumped had been the top of a cliff before the rise in water level after the ice age. I should have remembered his resentment of pre-historians and kept speculation to myself.

‘That straight edge could go down below the mud,’ I said, ‘and the chances are that the ice floes undercut it at the bottom – a useful shelter for the hunter when the climate began to warm up. Salmon by the million and the odd mammoth when he was sick of fish. With a couple of skins hanging down from the roof to keep the wind out and a good fire of driftwood, he was quite as comfortable as any Canadian Indian.’

‘You believe that one could still find traces of such shelters?’ Marrin asked.

I replied that nobody would take the trouble to dig down through fathoms of mud just to find a hearth and some worked flints. But wherever there was a clean Severn cliff underwater the skin-diver might well find some evidence that man had lived at the foot of it or under it.

‘I think you were right when you told me that no such cliff remains,’ I added. ‘The only possible site would be the Shoots at the entrance to the Bristol Channel.’

We drove quickly back to Broom Lodge, and after we had warmed up in hot baths he invited me up to his laboratory for a drink. There I was to see another side of him, undoubtedly more genuine than his impostures such as alchemy.

‘You must have been the only person to have explored the bottom of the Severn,’ I said.

‘In our time I may be. But long ago when, as you tell me, the sea was lower and a calm Severn flowed out to it, there could have been another. You have seen the Temple of Nodens?’

I had not, though I knew of the mosaics and the inscriptions.

‘The god of the river and the forest, father of Gwyn ap Nudd, prince of the underworld,’ Marrin said. ‘He was worshipped by Briton and Roman but he was here long before either of them. It might have been he who taught the forest-dwellers how best to fish for salmon and how to find veins of iron and gold under the dead leaves.’

‘And what is he reincarnated as?’ I asked. ‘Harbourmaster at Sharpness Docks?’

‘That is quite possible,’ he answered without at all resenting my levity. ‘Or myself perhaps. Or just a presence of whom I am aware.’

Spirits about. I was surprised, but it was to be expected. Some of the Buddhist sects do not, I believe, exclude the existence of good and evil spirits.

To change the subject, I asked whether he had found his curious turtle in the Severn.

‘In the deep, yes.’

‘You should show it to a zoologist.’

‘Do you know one?’

‘Several. And one of them is an authority on reptiles. I’ll bring him to inspect it.’

It was at that point, I think, that he finally made up his mind.

‘I am going to dive tomorrow night. Why not come with me? I have always wanted a companion.’

‘On another visit I’d like to. But I must leave tomorrow. Nobody knows where I am.’

‘I remember you telling me. But does nobody care, my dear Piers?’ he asked, using my Christian name for the first time. ‘You left no address?’

‘No. Letters can wait.’

‘And where are you going?’

‘I’ll have a look at your Temple of Nodens and then on into Wales.’

True enough, but the real reason was that I wanted to escape. Broom Lodge, the major, Marrin himself, Elsa, and that silent and ancient forest, so unnecessarily mysterious, were beginning to form a whole which I distrusted. I needed a few days alone in which I might isolate Elsa from the rest and decide whether or not I was making a fool of myself.