Now came the real test. Deeper and deeper, Tom cleaved his way downward. Reaching bottom, he prowled about the ocean bed for a while, then started up again. Suddenly a stab of pain shot through his chest--a warning of nitrogen bubbles forming in his blood! Tom swam toward the signal cord, dangling dimly in the distance. By the time he reached it, his muscles were knotting with cramps. "It's the bends again, all right!" Tom realized. Gritting his teeth, he yanked hard on the line, then summoned his strength to hang on. Doc and Chow hauled up frantically. Tom's face was contorted with pain when they finally got him aboard and stripped off his mask. "Oh! How awful!" Phyl gasped. Sandy cradled Tom's head in her lap, and Phyl held his hand sympathetically, while Doc Simpson injected a hypodermic to ease the pain. Chow steered the launch back to shore, and Tom was rushed to the base infirmary in an ambulance. Here he was placed in a decompression chamber for several hours and later transferred to a hospital bed. Bud Barclay came to visit him. "We're a fine couple of fish," he said. Tom chuckled wryly. "_Live_ fish, anyhow." "In my case, thanks to you," Bud said. "Forget it, pal. The score's about even, I should think," Tom said, recalling the many life-or-death adventures they had shared. Bud was thrilled to hear of Tom's electronic hydrolung. The young inventor spent the evening sketching out an improved design to eliminate future accidents. "I'll install a special device to remove the nitrogen as the wearer exhales," Tom explained. "Then a valve will feed in helium to replace it. Since helium doesn't dissolve in the blood like nitrogen does, it will not bubble out when the pressure is reduced. Should have thought of that before!" "But you'll need a tank for the helium, won't you?" Bud objected. Tom shook his head. "Enough can be compressed into a small capsule to supply the wearer's needs. Remember, it can be used over and over again." "Pretty neat," Bud commented. By morning Tom felt thoroughly recovered. He insisted upon flying back to Enterprises to make the necessary changes in his hydrolung. Bud accompanied him, eager to get back on the job. In a few hours Tom had added a small fitting to his power unit to provide for helium substitution. Then the two boys hopped back to Fearing for a second deep-water test. This time, Tom was delighted to find that he could operate comfortably at great depths, as well as rise or descend suddenly without ill effect. Bud was aglow with enthusiasm. "Boy, we can really explore now!" After the boys had returned to Enterprises, Tom phoned Arv Hanson and asked that a duplicate of the hydrolung be turned out in the shop as soon as possible. It was ready the following Monday morning, so Tom suggested to his father that the two visit the proposed underwater site and make some sample plantings.