I inched along the sea-bed. It remained smooth, like skin. Light filtered from above, muted as by a blind drawn against the sun. The floor sloped slightly upwards. There was no rip or, surge as I had expected near the mouth. I glanced left and right. Nothing.
I faced ahead. My limbs seized. A cupola had risen swiftly on the ocean floor, bulging, apparently sensate. It undulated, igloo-high, towards me. My limbs wouldn't kick me clear. My right hip banged against it. Soundlessly it vanished. Floating free, the size of a radio-sonde balloon, a bubble rose slowly past me.
Gas!
I pulled up my legs and sat down, shaking with reaction. There was, as Rhennin had asserted, a simple and rational explanation for the whole thing: here now before my eyes. The blood-red sunsets, the lavish displays of phosphorescence, the warm mugginess of the day, the lack of marine life, the green mud, the Bells — all fell into place. What a fool I had been! I was in the presence of that rarest of sea phenomena, what scientists call an azoic area — a sea without oxygen and therefore without fish. The sea is robbed of its life-giving oxygen by a sulphate-reducing type of bacteria which changes harmless sulphates normally present in sea water into an evil-smelling gas. Millions of decaying organisms are the primary cause of the gas: the sea round Mercury must be filled with untold numbers of them. Relief sharpened my mind as I was able to explain it in rational terms: the green mud was, in fact, a layer of billions of these dead daitoms, or common microscopic sea plants. The textured mud I had fled from was composed of countless skeletons of these invisible creatures." When cold currents swept the sea-bottom, bacteria-producing pockets of gas accumulated in the green mud. When a warm counter-current flowed — brought about by the north-west wind instead of the prevailing south-wester — the pockets of sea-bed gas burst, bubbling up and destroying all fish life.
It seemed to explain the Bells: simply a low heavy rumbling as the pockets of sea-bed gas escaped. It also explained why the Bells occurred only at irregular and intermittent intervals. As for the gas itself — my mind raced in relief at the natural explanation which robbed Mercury of its terror — it was sulphuretted hydrogen which, although smelling repulsive, was quite harmless to humans, although fatal to fish…
I stared in surprise, at the thin extra signal line which dangled overhead. Odd, that Rhennin should have sent down another. Odder still, it was floating loose, with no weight attached. My own rope was still fast to my wrist. My decompression meter told me I had another quarter of an hour below. I decided to ignore the drifting white nylon cord and concentrate on making a quick prospect of the sea-bed. If there were diamond gravel it would lie under the layer of mud, which I hoped was not more than a few inches thick. I scooped and scratched a patch about three feet square. It was soft and easy to work. In a few minutes I felt the rough thrill of gravel beneath. Diamond gravel! I used my knife and filled a small canvas bag at my belt. As assay of so small an area was not a fair assessment but the fact that within minutes I had found diamond-bearing gravel was encouraging. The Mazy Zed's pumps would make light work of the mud.
The new signal line drifted down almost into my prospecting hole, wandering and waving. I reached out to give Rhennin a reassuring tug; I stopped short. If it were not weighted, how did it stay down? Rope does not have negative buoyancy! The weals on Pieterse's neck! It was wavering tentatively towards my shoulder. Irrationally, blindly, I kicked out of reach. I had gone only a few yards when I pulled myself together. My nerves would be shot to hell if I accelerated like a ramjet at anything unusual… the white line drifted upwards. It was too thin for an octopus tentacle, and seemed to be uniformly so for its whole length. I followed the undulating thing. I glanced at my compass. My first suspicion hardened when I saw the line did not reach towards the surface higher than the seven-fathom mark. It turned eastwards, almost horizontal. Eastwards — to the mouth of the Glory Hole!
I switched off my powerful waterproof torch so as not to reveal my position to whatever lay ahead. It was difficult to pick out the line by the water-filtered light, so I turned over on my back, following its course. Then the light brightened from refractions inside the cavern, as in the Blue Grotto at Capri. I edged a little deeper. If there was anything there, I wanted to see it first, before it saw me. The white line was below me. I had left it above! I glanced upwards. There wasn't one, there were half a dozen lines, all floating parallel, quartering the sea. They seemed without life, without function.
I planed eastwards.
Then I saw: the lines ended. They came from an opaque, gelatinous screen through which the light, reflected from the cavern, shone with pearl-like iridescence tinged with translucent blue. I edged closer, my heart racing. The white lines trailed and twisted.
I knew what had killed Pieterse.
The curtain in the water ahead was a living creature.
It was a composite of thousands of jelly-fish. The white trailers were not rope but deadly thread-cells or cnidocils, loaded with a fatal chemico-toxic sting allied to a stunning electrical discharge. It was a rarity any marine scientist would have given his left hand to see.
I edged still closer: it looked like a screen in a pub made of empty bottles fused together. A huge colony of the individual jelly-fish or Portuguese men-o'war which float by the acre off the Sperrgebiet. They are known as siphonophora. Being such a lowly form of life, they would not be affected by the gas as fish were. Each jellyfish is a perfect individual, complete with mouth, stomach, sexual organs, propulsion apparatus and a sting powerful enough to kill small hard-shelled creatures and produce intense burns in humans. The Sperrgebiet jellyfish has the unique capacity of being able to undergo what scientists call symbiosis: by this process individuals link together into a colony and in doing so they surrender individual functions to the composite creature. One group of thousands forms a complicated apparatus for propelling the new body; another forms its sexual organs; another the stomach and digestive tract; another the liver; and yet another the killer-sting. In pooling themselves, the individuals lose all but the function they dedicate themselves to in the new animal. It becomes a complete new organism. And in the gross example before my eyes the sting had developed rope-like proportions whose touch was immediate death. Pieterse had brushed one. Sensitized, it had found a tiny crack where the hood joins the suit, hence the weals on his neck. I knew now why Shelborne had never been able to dive into the Glory Hole. It must have taken years for the colony to grow and develop to such bizarre proportions.
I glanced at my decompression meter. Only eight more minutes! I dived deep, away from the screen, down to the green mud. I peered through the shimmering, distorting, filtered light at shapes I could see vaguely looming there. There was no mistaking the outline of the lean hulls — Gruppe Eisbar,' Three were moored alongside one another and farther inwards a blur could have been the other two. The screws and hydroplanes of the nearer boats were fuzzy but distinguishable. Goering's cache was as safe as in Fort Knox. I checked again on my meter: I should already have been on my way to the surface. I swam away from the stings and gave four signal tugs — pull me up!