The first few steps were the worst. Once I could fashion a grip for my hands, however, the work went quickly. It was mercifully cool and the smell of water was tantalizing. Although it grew darker, there was some suffused light deeper down. After I had chipped foot- and hand-holds for about fifty feet, I sent Koeltas down for his stint. I lay in the shadow of the cairn.
Among the savage gramadullas I thought I saw a helio of light. Shelborne was watching us.
I cut the final steps. Down, down, down. Then I saw: below me was water — swift, flowing, with chocolate reflections as from polished steel. The shaft was through the roof of an immense cavern. A beach of pure white sand would cushion my drop of the last twenty feet. I shouted to Koeltas to come, then let go and fell.
I lay still, spellbound by the muted loveliness of the scene after the torture of the dunes. We were in a huge cavern to whose dim roof soared enormous pillars of limestone, intercalated with hundreds of pure white stalactites. We lay and cupped the water to our mouths. I estimated the river to be half a mile wide; it may have been more. The white beach was littered with Bushmen bones and skulls, and ran along the water's edge into the distance.
Koeltas anticipated my thoughts. 'It flows to the sea! To Mercury!'
My hunch had been correct: here was the old river which had once flowed in the bed, now dry above, and had been forced into this subterranean channel by the uplift of the coastline. Here was the diamond-carrying river, the distributor of the fountainhead's riches! The fountainhead itself must lie between Hadje Aibeep and Mercury. There was nothing ancient, however, in the strong young flow of the river. The brown told me, this is floodwater. Was it on its way to burst through to the sea like the Orange at Oranjemund? There was no mouth in Spencer Bay; the outlet must also hold the secret of the fountainhead. We must follow the river, here where we could see.
Our march was cool and easy after the aridity of the dunes. The sand fringe was pebbly and comforted our blistered feet. We rejoiced in the smell of water. The shaft of Hadje Aibeep illuminated the first part of our way. Then Koeltas grew uneasy about the dark, but it seemed obvious that the line of what I had thought to be blowholes must be vents similar to Hadje Aibeep, though smaller. I had not expected them earlier than Strandloper's Water, but there were some at irregular intervals. Above, they must lie among the wasteland of rocky outcrops we had avoided, which would account for our not having seen them.
By nightfall — the cavern became pitch-back as the sun sank — I estimated that we must be half-way back to the coast. Before rolling in my sleeping-bag, I went to the water's edge. The water had ceased its rapid flow and was sullen, turgid, scarcely moving. What mammoth obstruction lay ahead to dam it up? It also seemed the the cavern's roof was lower — was the water-level rising towards the ceiling? The quantity pouring in would trap us if there were no exit, and the occasional vent was far out of our reach. To climb the stalactites would be as impossible as to scale a skyscraper. Was Shelborne's killer in front? Or behind?
By mid-morning next day we passed, according to ray dead-reckoning, directly under Strandloper's Water. The roof was lower and the water, so friendly the previous day, was menacing. The river beach had disappeared about Strandloper's Water and now we picked our way cautiously along water-smooth rock within a couple of feet of the river itself. Ahead — maybe half a mile — I saw a shaft of sunlight. I made up my mind.
'Koeltas,' I said, 'if that next blowhole is close enough to our heads, let's get out of here.'
He shuddered and nodded. As the ceiling had crept nearer our head, he had become more strained. From time to time he looked back, holding the rifle.
We were in the sunshaft of the blowhole. I hauled Koeltas on to my shoulder, but it was still out of reach. The smooth rock offered no hand-holds. We would have to go on. What had killed the springbok at these blowholes?
The going worsened and we found ourselves clinging in places to the wall and splashing in the water. Ahead, another shaft cut into the blackness. I edged forward. My head touched the roof. A thrill of panic ran through me, blind, unreasoning fear. Where in God's name were we? We were trapped like rats in a sewer. Now was the time for Shelborne to make his kill. The water was deep, marginless, on my right. What if we emerged in the quicksands? We would simply be exchanging one form of dying for another. We must get out!
The thunder of the crash stunned me — heavy, sudden, like a broadside from ahead, an ear-deadening, diaphragm-ripping bolt of sound.
The Bells of St Mary's!
Cringing on the rocky shelf, there was no barrier of sea or Mercury or rockroof to cushion us: we were in the combustion chamber of the Bells.
I grabbed wildly. My grip went into Koeltas's thin shoulder. His slitted eyes were wide with terror, I opened my mouth, but I could not hear my voice. We must get out, get out quick, if we wanted to live.
I gestured forward to the blowhole. We tore, scrabbled, fought our way, ripping our clothes, bruising our knees and palms, tearing our fingernails. Half-way to it the water rose, rose, rose. I abandoned the rock and swam, striking out desperately. My knuckles struck the rock-roof as I flailed. My kick grated against the steel of the Remington — Koeltas swam with it slung. Half a dozen strokes to go. Then I saw.
The turgid chocolate surface, mocking a surfacing U-boat, swelled like the sea-bed off Mercury.
Gas!
My brain registered while my body fought — in slow-motion it seemed — for the opening.
Diamonds… blowholes… gas!
I lived again, in mental photoplay, a scene years before: the same dark tunnel, the black roof, the vent. Jagersfontein, one of the big old diamond mines not far from Kimberley. There had been a fissure in the rock — but it had burned. And it had burned for thirty years, they told me.
Burning gas was methane gas — killer-gas!
Methane gas — not the harmless rotten cabbage gas whose bubbles I had seen on the sea-bed — was bursting through the swollen river. The oxygenless sea was quite a separate phenomenon from that in front of me now. The solution to the whole complex problem fell into place. There were, in fact, two gases: one which I had found on the sea-bed which was fatal to fish and harmless to humans, and the second which was fatal to everything. Methane had killed the great herd of springbok, forcing its way through blowholes and smaller fissures where they scented the underground water. The harmless gas was found only at sea, but the killer-gas was present on the land and on Mercury. That was the Bells! — methane, alias the Bells! The Bells were not regular but intermittent since it required a build-up of pressure for the gas to burst out. Another great piece of the jigsaw had fallen into place.
Instead of a barrier of sandbars, like Oranjemund, I knew now that the ancient river now in flood was held back from the sea not by sandbars but by pressure of methane gas. The whole coastline and bay under Mercury must be a gigantic cupola-shaped underground cavern, of which the Glory Hole was a small exit. The cupola, shaped like an inverted steel helmet, was filled with methane gas. The river flooded and debouched, into the domed undersea cavern — the diamond fountain-head itself! Enormous pressure built up as millions of gallons of water compressed millions of cubic yards of methane gas. More water poured in; more gas was accumulated. Pressure built up and up. The rockroof under Mercury took a strain like a hydrogen balloon in the stratosphere, hundreds of pounds to the square inch. The roof trembled under the great pressure — and Mercury shook. It took years for the Orange River floods to work up sufficient pressure to break the sand barrier; it would take this underground river as many years to do the same to Mercury. When the Oranjemund sandbars burst, they said, you could hear the noise for more than a hundred miles out to sea. A British warship during the war had cleared for action, thinking the Orange's breakthrough was gunfire.