‘Then if you are in no hurry, come with me now! The tide will serve tomorrow night and might not on your next visit to us.’

‘But why not daylight?’

‘Oh, you should know that! At night the strangest creatures come out of their holes and swim freely. And with a torch at night one sees colours as never in daylight. The kingdom of Nodens – doesn’t it tempt you?’

It did. I was eager to see the glow of the weed and the silver offish against the red and green marl of the Severn.

‘Good! And may I ask you to tell no one you are going with me? Not even Elsa.’

‘But why not?’

‘They would be jealous and pester me until I started diving classes. You’ll agree that the Severn is no place to learn. And there is another point; as between friends I can admit it to you. If they knew that I allowed you to go with me, they would see my night dives as a mere sport.’

‘Instead of a mystery of mysteries?’

For a moment I thought I had gone too far, but the blaze in his eyes was instantaneously extinguished. He replied quite calmly:

‘To be the leader of a faith one must offer the followers secrets for their imagination as well as truth for their souls.’

Never was there a more curious reason for skin-diving at night, but his frankness was convincing. He himself apparently found a unity with nature in these driftings far below the swimming of the salmon. That in itself was a pleasure not far from spiritual, so in his own eyes he was justified in presenting it as an aspect of religion, perhaps linked in some way to the more dubious alchemy.

I said that I didn’t see how I was to prevent the commune knowing that I had left with him for the river.

‘Oh, that’s easy! Leave in the afternoon to continue your walk as you intended. At ten be on the Box Rock. You can reach it from the road to Awre. We will change there, and it is only a few yards to the ledge above my favourite deep where we will dive.’

‘Where can I warm up and stay the night if I don’t come back to Broom Lodge?’

‘Leave it to me, Piers! I’ll find accommodation for you at one or other of the nearby inns and tell them to expect you late.’

I felt it was absurd not to discuss his invitation with Elsa, and it is likely that I would have done so if ever we had been together long enough for a private conversation. Neither she nor I could risk venturing down the passages and through the main building when a party of the faithful might be meditating with open eyes, others getting up before dawn and saying good-night or good-morning to a prayer-meeting in the hall, or the major working out Arthur’s past or future tactics on the lawn. Bedroom-creeping, however cautious, was out; the movements within Broom Lodge were incalculable.

So Elsa and I were limited to quick kisses in corridors, with no chance to talk seriously about the future. She may have feared it and wished to avoid it. When I left in the afternoon I promised to return as soon as I decently could and meanwhile to send a letter or two to remind her of what I thought of her. She wanted to know where I would stay the night and I told her that I was going to Lydney to look at the Temple of Nodens and did not know where I should fetch up afterwards – both of which were true. She insisted that I should take cider and sandwiches with me so that I could have a meal, without thinking of time and communications, wherever I found something worth detailed investigation.

I set off about four o’clock after telling them all how grateful I was and how interested I had been, and walked through the green forest paths to the remains of the Temple set on its headland, which once must have projected into the Severn. Evidently Nodens had been one of the friendly little gods of the Celts. Marrin’s suggestion that in life he had been a hero from across the seas, bringing agriculture or the working of metals, was a reasonable guess.

Then I took a bus up the river and walked down to the sea wall where the great horseshoe bend begins and the tide, compressed by the narrowing river, explodes into the bore. I reckoned that the wave should be at its best if there was light enough to see it, for it was the night after new moon.

I did not want to call at a pub and start eating and drinking before a dive. Marrin’s deep was unlikely to be more than twenty or thirty feet at low tide, but it was bound to be tricky. However, the hours of waiting did not drag. I sat on the low red cliff above the mud, watching the fearsome ebb racing down to the sea, not even sticking to the main channels of its bed but dancing in whirlpools, dashing up unseen backwaters against the flow and forming dark drifts of silt which compelled me to look up at the sky and see whether they were not the shadows of clouds overhead. But there were no clouds except over the distant Cotswold hills when the red sun went down behind the pinnacles of the Forest, leaving a warm night behind, perfect for diving.

I started to move towards the rendezvous while the long twilight could show me the way. The Box Rock ran out at a right angle to the shore, and part of it was now showing above the streaming ebb. If there was a clean and sheltered drop on the downstream side it was easy to understand why Marrin had called it his favourite deep.

The first I saw of him was the pool of light from his torch wavering over the meadow and the offshore mud. He had his suit on under a duffle coat and carried the aqualungs and the spare suit and life jacket for me. He helped me to dress fussily, exactly and with the utmost friendliness, meanwhile telling me of the likely conditions under water, that I should follow him closely and that my two cylinders would allow me some eighty minutes. We should come out, however, in less than half an hour, well before the turn of the tide.

At about ten-thirty we were on the rock and ready. He walked downstream until the water was nearly up to his knees and stopped.

‘We’ll jump from here,’ he told me, ‘into the Box Hole. Another step and I should be over the edge of the cliff.’

The dark surface of the water was disturbed for no apparent reason; I could see no other sign of a sudden increase in depth. When I jumped I fully expected to land on my bottom, but found myself easily descending along the face of rock. The tide was hardly perceptible and the water less opaque than I expected, so that the colours of Severn rock could be distinguished. Marrin kept close to me and a little ahead, his lamp showing me what to look for. Once a conger trailed out of a fissure in the cliff and passed upstream ahead of us so that one could watch the long, silver undulations, half fish and half snake. There was no weed except for occasional clumps.

When I looked for Marrin he had gone – in pursuit of the conger, as I thought, or perhaps out into the channel beyond the rock. Finding that I was negatively buoyant I started to walk along the bottom of the Box Hole and didn’t much like it. The bottom was quicksand or some yielding emulsion of mud and sand into which the fin on my right leg sank. The effort of pulling it out broke the strap and of course drove down the left leg. Cursing Marrin for not seeing that the strap was in good condition, I recovered the fin, but it would not stay on and was useless. I pulled out the left leg with some difficulty and decided that I had had enough. The silt stirred up by my efforts blinded me, and I no longer knew where the rock was. I was experienced enough not to panic, for I had only to release the weight belt round my waist and come up. I didn’t give a damn if his weight belt was lost for ever in the sand, as it certainly would be.

I felt for the release catch but it wouldn’t release; it had jammed. But it couldn’t jam! Then I did panic – it had not jammed. It had been jammed – and cunningly, for I couldn’t see or feel how. Meanwhile, the weight of the cylinders was pushing me little by little down into the quicksand. My right leg was kicking to no purpose. My frantic efforts to clear my left leg broke the strap on that fin also.