In 1925 the geological connection between Flint Ridge and Mammoth Cave was proved when dye placed in a Flint Ridge spring showed up in Echo River at Mammoth Cave.   That tantalizing swirl of dye confirmed specula­tions that were to tempt more than 650 cavers over half a century with the thrill of being the first to make human passage of the cave connection. Roger Brucker and Richard Watson tell not only of their own twenty-year effort to complete the link but the stories of many others who worked their way through mud-choked crawlways less than a foot high only to find impenetrable blockages.   Floyd Collins died a grisly death in nearby Sand Cave in 1925, after being trapped there for 15 days. The wide press coverage of the rescue efforts stirred the imagination of the public and his body was on macabre display in a glass-topped coffin in Crystal Cave into the 1940s. Agents of a rival cave owner once even stole his corpse, which was re­covered and still is in a coffin in the cave. Modern cavers still have a word with Floyd as they start their downward treks.   Brucker and Watson joined the parade of cavers who propelled themselves by wiggling kneecaps, elbows, and toes through quarter-mile long crawlways, clinging by fingertips and boot toes across mud-slick walls, over bottomless pits, into gur­gling streams beneath stone ceilings that descend to water level, down crumbling crevices and up mountainous rockfalls, into wondrous domed halls, and straight ahead into a blackness inten­sified rather than dispelled by the carbide lamps on their helmets.   Over two decades they explored the passages with others who sought the final connection as vigorously as themselves. Pat Crowther, a young mother of two, joined them and because of her thinness became the member of the crew to go first into places no human had ever gone before. In that role, in July 1972, she wiggled her way through the Tight Spot and found the route that would link the Flint Ridge and Mammoth Cave systems into one cave extending 144.4 miles through the Kentucky limestone.   In a new afterword to this edition the authors summarize the subsequent explorations that have more than doubled the established length of the cave system. Based upon geological evidence, the authors predict that new discoveries will add an­other 200 miles to the length of the world' s longest cave, making it over 500 miles long.