THE intriguing secrets and mysterious extinction of dinosaurs are the subjects of an exhibition currently running at the Science Museum. ''Through life-like robotic dinosaurs, multi-media shows, computer games and hands-on exhibits, 'Dinosaurs Alive!' will recapture the life and times these gigantic prehistoric creatures,'' Urban Council Vice-chairman Mr Lo King-man said. ''Worldwide interest in dinosaurs is intense. Not only because they are the largest of all land-based creatures that ever lived, but because of the great unsolved mystery of their disappearance 65 million years ago.'' Sounds and lighting effects associated with each of the eight dinosaurs on display are reproduced electronically, and movements, which are also computer-controlled, are achieved by a system of values and pistons using compressed air. . In addition to viewing these beasts, visitors can also act as paleontologists by digging up fossils with tools provided, take a dinosaur skeleton for a walk and design their own dinosaurs on a computer. ''Dinosaurs Alive!'' runs until March 13, 1994 in the Special Exhibition Hall of the Hong Kong Science Museum. Admission is $10 (adults) or $5, in addition to the $25 or $15 entry charge to the Museum.