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Dr. Todd, he was reassured to observe, always kept within arm’s length; he was being almost ridiculously overcautious. Even if a shark came shooting up out of the depths, Duncan believed he could be aboard the dinghy in two seconds flat-notwithstanding Earth’s gravity.

Now that he had mastered the use of the snorkel tube, he kept his head under water all the time, and even essayed shallow dives which involved holding his breath for considerable periods. The panorama beneath was so fascinating that Duncan even occasionally forgot the need for air, and emerged sputtering foolishly.

The first signboard was at a depth of five meters and said, in fluorescent yellow letters: NO UNAUTHORrZED VISITORS BEYOND THIS POINT. The second warning was a flashing holographic display in mid water which must have been very perplexing to the fish. It announced ominously: THIS REEF IS

MONITORED. Duncan could see no trace of the projectors; they had been very cunningly concealed.

Todd was pointing ahead, to the line of divers working along the edge of the reef. So he had not been joking. They really were going through the unmistakable motions of gardeners digging up noxious weeds. And each one was surrounded by a small cloud of brilliantly colored fish, clearly benefiting from all this activity.

The coral formations seemed to be changing shape. Even to Duncan’s untrained eye, they looked strange-even abnormal. He had grown accustomed to the branching antlers of the stag-horns, the convoluted

labyrinths that looked like giant brains, the delicate 213 mushrooms sometimes meters in diameter. They were still here, but now subtly distorted.

Then he saw the first metallic glint-then another, and another. As he came closer, and the blue haze of distance no longer softened the details of the underwater world, Duncan realized why this reef was cberisbed and protected.

Everywhere he looked, it glittered and sparkled with gold.

Two hundred years earlier, it had been one of the greatest triumphs of biological engineering, bringing world fame to its creators. Ironically, success had come when it was no longer required; what had been intended to fulfill a vital need had turned out to be no more than a technological cul-de-sac.

It had been known for centuries that some marine organisms were able to extract, for the benefit of their own internal economies, elements present in seawater in unbelievably small proportions. If sponges and oysters and similar lowly creatures could perform such feats of chemical engineering with iodine or vanadium, the biologists of the 2100’s had argued, why could they not be taught to do the same trick with more valuable elements?

And so, by heroic feats of gene-manipulation, several species of coral had been persuaded to become gold miners. The most successful were able to replace almost ten percent of their limestone skeletons with the precious metal. That success, however, was measured only in human terms. Since gold normally plays no part in biochemical reactions, the consequences to the corals were disastrous; the auriferous reefs were never healthy, and had to be carefully protected from predators and disease.

Only a few hundred tons of gold were extracted by this technique before large-scale transmutation made it uneconomic; the nuclear furnaces could manufacture gold as cheaply as any other metal. For a while, the more accessible reefs were maintained as tourist attractions, but souvenir hunters soon demolished them. Now only one was left, and Dr.

Mohammed’s staff was determined to preserve it. So, at regular intervals, the doctors and nurses took time off from their usual duties, and enjoyed an arduous working holiday on the reef. They dumped carefully selected fertilizers and antibiotics to improve the health of the living corals, and waged war against its enemies-particularly the spectacular crown of thorns starfish and its smaller relative the spiny sea urchin. Duncan floated, perfectly relaxed, in the tepid water, lazily flippering from time to time so that he remained in the shadow of the dinghy. Now he understood the purpose of those sinister knives and spikes; the adversaries they had to deal with were well protected indeed.

Only a couple of meters beneath him, one of the divers was jabbing at a colony of small black spheres, each at the center of a formidable array of needle-sharp spines. From time to time one of the spheres would be split open, and fish would dart in to grab the pieces of white meat that came floating out. It was a delicacy they could scarcely ever have enjoyed without human intervention; Duncan could not imagine that these spiky beasts had any natural enemies.