Down they glided into the sea-green wilderness. Leveling off in sight of the ocean floor, they tried their drive jets. The effect was thrilling! _Zip ... Whoosh!_ They darted to and fro like human torpedoes. Then Tom twirled the control knob of his density unit. Immediately he bobbed upward like a cork. A reverse twirl sent him plummeting toward the bottom again. Bud, watching with wide-eyed excitement, began experimenting on his own. Soon the boys were engaging in all sorts of underwater acrobatics. Presently Bud felt a nudge in the back that sent him hurtling a dozen yards through the water. "Snuck up on me, eh, pal?" he thought with a chuckle. "Okay, Tom old boy, here's where the undersea terror strikes back!" Swooping around to return the compliment, Bud gulped in surprise. Instead of his chum, he found himself face to face with a bottle-nosed dolphin! "Good night!" Bud thought. "A porpoise! So you're the joker who nudged me!" With a playful toss of its comical-looking snout, the porpoise swam off, as if inviting Bud to join in the fun and games. A whole school of the creatures cavorted into view. "Okay! If you want to play!" Chuckling, Bud darted in pursuit, whacked the porpoise that had nudged him, and jetted off again. The porpoise gave chase, whistling and grunting audibly. Tom joined in the fun, and soon a rollicking game of underwater tag was in full swing. The dolphins seemed as playful and mischievous as small children. Twenty minutes later the boys surfaced and hauled themselves aboard. Both tore off their masks and flopped into the boat, shaking with laughter, surfacing and diving. "What was so funny down there?" Zimby asked. When Tom told him about the dolphins, he too burst into laughter. The porpoises rose into view and convoyed the launch all the way back to the island. The boys were so jubilant over the performance of the new hydrolung gear that Tom decided to press his search for the Brungarian sea-prowlers immediately. Soon after lunch they took off in the _Sea Hound_ and headed for the South Atlantic. Hank Sterling, Chow Winkler, and two crewmen accompanied the boys. Dazzling afternoon sunshine sparkled over the sea when they reached the missile search area. Tom immediately contacted Art Wiltessa and the task-force ships. They had no new developments to report. The young inventor gave orders to submerge. As soon as the seacopter touched bottom, Tom and Bud swam out through the air lock with their hydrolungs. They probed about for half an hour, ranging farther and farther from the _Sea Hound_. Then Tom felt a touch on his arm. He turned and saw Bud pointing off excitedly to the right. A strange submarine was moving slowly toward them! CHAPTER VIII DATE TROUBLE The boys exchanged looks of fear through their face masks as the knifelike hull and conning tower of the submarine loomed gray and ghostly.