By Michael Crichton

Knopf. 385 pp. $17.95

MICHAEL CRICHTON is a medical doctor turned writer and film director whose first book, The Andromeda Strain, was a best-selling novel about a virulent organism of extraterrestrial origin that threatened life on earth. Sphere is similar in that an extraterrestrial presence is again involved, but this one lies 1,000 feet below the surface of the Pacific Ocean, where a gigantic spacecraft has been discovered. The U.S. Navy, by measuring the coral encrustations on it, estimates it has been there for at least 300 years.

A team is assembled to descend to an underwater habitat placed next to the strange craft. In addition to the naval commander and support staff, a group of civilian scientists are included. Ted Fielding, an astrophysicist, seems to worry more about quotable quotes and desirable camera angles than he does his science. Harry Adams, a black and a mathematician, is cocksure, impatient, arrogant -- and very smart. Beth Halpern is a weight-lifting biologist who vacillates between feminist hostility and a kittenish sexuality. "Mother Nature with muscles," according to one of her colleagues. But Norman Johnson is our main man, a 53-year-old psychologist not too keen on the idea of being locked up 1,000 feet down. He's a bit too much of a nerd for my taste, given to explaining things in such terms as "people who aren't in touch with their emotions tend to think that emotions are unimportant." Norman starts off as a detached and somewhat supercilious observer, but when the wild things begin, he's forced into the middle of the action.

Unlocking, entering, and searching the spacecraft is like opening a series of nested boxes, each exposing another riddle. The final one is a "large, perfectly polished silver sphere about thirty feet in diameter." How to open it?

Having a spaceship immersed in water gives the author an opportunity to explain the widest range of scientific and technological issues, everything from black holes in space to the effect of pressure on the human body. Crichton is adept at it, especially when he can call upon his medical training, and he uses imagery the lay person can easily follow. For example, at very high pressure, "the atmosphere of the Earth is deadly. You don't realize it, but oxygen is a corrosive gas. It's in the same chemical family as chlorine and fluorine, and hydrofluoric acid is the most corrosive acid known. The same quality of oxygen that makes a half-eaten apple turn brown, or makes iron rust, is incredibly destructive to the human body if exposed to too much of it. Oxygen under pressure is toxic -- with a vengeance. So we cut down the amount of oxygen you breathe. You breathe twenty-one percent oxygen at the surface. Down there, you breathe two percent oxygen."

These details, rather than the characters, make this book seem more believable. Although toward the end I did become sort of fond of old Norman, most of the others seemed cut out of cardboard. In particular, Crichton's treatment of the captain of the habitat annoyed. The man is stupid as an ox and spends most of his time screaming out orders at the top of his lungs. Apparently, all he cares about is discovering a new weapon for the Pentagon. Too stereotypical, not believable.

The plot? Well, that's the key to the enjoyment of this book: whether you can go along with it, or find it just too much. I don't want to tell enough to spoil it, but would you believe that attacks by giant squid are the least bizarre of the fast and furious onslaught of emergencies that confront the four scientists? Early in the book, Crichton leavens it with a bit of humor, such as the time prior to the descent when one of the bureaucrats explains his plans for any eventuality: "You realize the President will want to talk to these aliens personally. He's that kind of man . . . The President meets with the aliens at Camp David. What a media moment!" Reassuring, isn't it? But toward the end, there's no time for fun, its all WHAM! BOOM! BASH! for Norman and company.

It's a short book, and the pages turn quickly. For a little while, suspend your disbelief, and put yourself 1,000 feet down. Suddenly, your computer terminal lights up, and there is a message that emanates from the spacecraft: "I am coming," it says . . . :: Michael Collins was command module pilot of the Apollo 11 mission to the moon and the first director of the National Air and Space Museum. He is the author of "Carrying the Fire."