"All right," he said, starting to strip.

"Wait a minute! Are you fellows crazy?" Malmstrom and Rice yelled almost together. "If that water's been killing the fish, you're foolish to go in."

"It's safe enough," said Bob. "We're not fish." He was aware of the weakness of this argument but could not think of a better one on the spur of the moment. The two Kenneths were still expostulating as he slipped feet first into the pool, with Norman beside him-both knew better than to dive into a coral pool, however clear it might be. Colby, who had not contributed to the argument, walked over to the boat, got an oar, came back, and stood watching.

The trouble with the pool manifested itself with remarkable speed. Bob swam out to the middle and did a surface dive, a maneuver which should have taken him without effort to the bottom eight feet down. It did nothing of the sort; his momentum barely got his feet under water. He took a couple of strokes, reached the bottom, broke a sea fan loose, and bobbed back to the surface with remarkable speed. As was his custom, he started blowing out air just before his head broke water, and managed to get some of the liquid into his mouth in the process. That was enough.

"Norm! Taste this water!" he yelled. "No wonder your fish died." Hay obeyed hesitantly, and grimaced.

"Where did all the salt come from?" he asked. Bob swam to the edge of the pool, clambered out, and started to dress before answering.

"We should have guessed," he said. Sea water comes in when the waves are high, enough, but the only way it leaves is by evaporation. The salt stays behind. You shouldn't have blocked off all the passages to the sea. We'll have to chip out one of your plugs and find some wire netting, if you still want to take those pictures."

"My gosh," exclaimed Hay, "and I wrote a school report on Great Salt Lake only last year." He started to dress, indifferent as Bob to the fact that he was still wet. "What'll we do? Go back for a crowbar or something, or poke around the reef for a while now that we're here?"

A brief discussion resulted in the adoption of the second plan, and the group returned to the boat On the way Norman pulled a large, battered bucket out of the bushes, laughing as he did so. "I used to fill up the pool with this sometimes, when I thought it was getting low," he said.

"I guess we can find some other use for Mr. Bucket now." He tossed it into the bow of the boat after the others, entered, and shoved off.

For an hour they rowed along inside the reef, occasionally disembarking on one of the larger islets, more often coasting alongside the ridges and lumps of coral while using poles to keep them off the more dangerous sections. They had worked some distance along the reef away from the island itself and had reached another fairly large islet-this one actually supported half-a-dozen coconut palms-where they disembarked and pulled the boat well up on the gritty soil. Their loot up to this time had not been very impressive, consisting mainly of a few cowries and a weirdly colored fragment of coral for which Malmstrom had gone overboard in twelve feet of water. The Hunter's profit from the expedition had been even less so far, which annoyed him, since the exploration of the reef for clues had been largely his idea.

He made the utmost use of Bob's eyes, however. They were nearing his arbitrary one-mile limit north of the beach, which meant that about half of the region in which he expected to find clues of his quarry's landing had been covered. There was still not very much to see, however. On one side of the irregular islet the breakers thundered; on the other was the relatively calm water of the lagoon, with the bulk of one of the great culture tanks a few hundred yards away. The scavenger barge was beside the tank at the moment, and the small figures of its crew were visible on the catwalks that crossed the paneled roof; beyond, and dwarfed by a distance of fully three miles, the houses of the island dwellers were barely visible.

These, however, could hardly be considered clues, it seemed to the Hunter, and he brought his attention back to the immediate surroundings. The present bit of land was similar to that on which Hay's pool was located, and like it, had very irregular edges-clefts, walled with living coral, in which the water gurgled down almost out of sight and then spurted upward into the watchers' faces as another breaker came thundering against the barrier. Some of the openings were narrow at the outer edge and broadened farther in, so that the water in them was quieter, though there was always the endless up-and-down wash started by the waves.

It was in these larger openings that the boys did most of their searching-it would have been impossible to get anything out of the others, reckless as some of the youngsters tended to be.

Rice, the first one out of the boat, had run to one of the largest while the others were still pulling the little craft up and making it fast; and dropping into a prone position with his head over the edge, he shaded his eyes and looked down into the clear water. By the tune the others came up he was already pulling off his shirt.

"My chance first," he said quickly as the others peered down to see what he had sighted. Before anyone was sure what he was looking at, Rice had slipped into the water, disturbing the surface so that nothing was clearly visible. He stayed down for some time, and finally reappeared asking for one of the poles that were carried in the boat. "I can't work it loose," he said. "It seems to be jammed in place."

"What is it?" several voices asked at once.

"I'm not sure; I've never seen anything like it. That's why I want to get it up." He received the pole which Colby handed down to him and slid under water once more. The object at which he was prying was about five feet below the surface-that is, it varied from about four to six feet as the water level rose and fell in response to the urge of the breakers.

Several times Kenneth came up for breath, without having dislodged the mysterious object; and finally Bob went down to help. He had one advantage over the other boy, thanks to the Hunter's prompt supplementing of the curvature of his eye lenses with some of his alien body material Bob could see much better under water than usual. He could make out easily the shape of the object on which Kenneth was working but did not recognize it. It was a hollow hemisphere of dull metal eight or nine inches in diameter and half an inch thick, with the flat side protected over half its area by a plate of similar material. It was hung on a stubby branch of coral only a few inches from the bottom, rather like a cap on a peg; and another lump of the stuff had either fallen or grown so as to wedge it in place. Rice was prying at the upper lump with a pole.

After a few minutes of futile effort they stopped, got their breath, and planned a more co-operative method of attack. Bob, it was agreed, would go to the bottom and work the end of the pole behind the object; Kenneth, upon receiving his signal, would brace one foot against the steep side of the pool-they both wore their shoes, as any sane person would in a coral pool-and push outward, to get the thing out from under the heavy fragment that pinned it down. The first time the attempt failed; Bob did not have the pole well enough set and it slipped out. The second, however, succeeded-almost too well. The piece of metal popped free and rolled away from the wall into deeper water; Bob, who was approaching the end of his store of breath, came to the surface. He refilled his lungs and started to speak to Rice, and then realized that the redhead was not visible. For a moment he supposed the other boy had taken a quick breath and gone back down for his prize, but as the water sank abruptly Rice's head appeared.

"Help! My foot-" The words were cut off again as the water surged up, but the situation was crystal-clear. Bob immediately dived again, braced his feet on the bottom, and strove to lift the lump of coral, which had been freed by the removal of the piece of metal and had landed on Kenneth's foot He was no more successful than before, and returned anxiously to the surface just as the water went down again.