Crashing waves, thundering avalanches, and hissing volcanoes are not the only sounds to be heard in the undersea world once thought to be silent. With the aid of hydrophones you can now hear snapping shrimps, squeaking lobsters, the ""drilling"" courtship songs of Chesapeake Bay croaking fish we mistook for German subs in 1942, and the ""foghorns and boat whistles"" a toadfish makes by vibrating its swim bladder. Many of the fish sounds seem to be forms of communication--to attract a mate or assert territoriality; yes, fish do have inner ears, and can also ""hear"" with their lateral lines of tiny sensors: Javanese fishermen have lured their catches by singing to them. But the noisiest sea creatures are the mammals. As for their communication, Cousteau has observed sperm whales heeding the call for help of an injured comrade, female porpoises about to give birth are known to summon ""nurses"" with a whistle, and killer whales have been discovered to have regional accents. Jacobs integrates ali these instances skillfully, explains the mechanics involved, and makes the subject so intriguing that readers who don't have hydrophones will probably create a run on the recently recorded ""Songs of the Humpback Whale."" The publisher's large-print format gives the book a young (third grade) look which we hope won't deter older readers from this readable and informative introduction.