Sphere by Michael Crichton (Knopf: $17.95; 385 pp.)

An American sailing vessel laying phone cable in the remote Pacific runs into a snag. Navy exploration reveals, buried on a shelf 1,000 feet beneath the surface, what appears to be a gigantic spaceship--completely intact, showing no signs of corrosion . . . and at least 300 years old. Investigators flown to the scene include a biologist, an astrophysicist, a mathematician, and a psychologist.

Startling questions arise from the very beginning: Is the craft alien or man-made? From our past, or from our future? And what is the nature of the mysterious hollow sphere they discover on board?

Crichton keeps us guessing at every turn, in his best work since “The Andromeda Strain.” Each chapter end reveals some new clue or poses some new threat that compels the reader to read on. And each new twist builds the pace with careful precision.

Precision is, in fact, a Crichton hallmark. His works give a sense of meticulous researching--no surprise, coming from a research MD. In “Sphere,” his details range from integral (The age of the space vehicle is dated by its coral growth: “Pacific coral grows two centimeters a year . . .”) to casual (In a habitat 1,000 feet underwater--30 atmospheres of pressure--you can’t make whipped cream: “Won’t whip”).

But such digressions are not merely entertaining in science fiction of this sort. They are essential to establishing an environment of believability, so that when the inevitable speculative leaps are made (and there are a couple big ones here), the reader eagerly tags along. Somehow it’s easier to buy the concept of extraterrestrial intelligence from an author who shows a little terrestrial intelligence himself.

Crichton shows plenty. Philosophical discussions abound, covering theories of extraplanetary life, black holes, human knowledge and behavior. The latter is actually a recurring theme, explored from the point of view of psychological Norman Johnson--the man who chose the other mission specialists, yet who remains under constant attack by them for being a champion of the “soft science” of psychology, in the face of their death-struggle against the considerably “harder” forces of inexplicable deep-sea monsters.

This whole set-up invites comparison to “The Andromeda Strain”--in which another group of scientists, cloistered in another isolated deathtrap, confront an extraterrestrial virus. Crichton himself raises the memory of that encounter early on in “Sphere”: “The fears unleashed by contact with a new life form are not understood. . . . But the most likely consequence . . . is absolute terror.”

Terror--the terror of death, and of the essentially unknowable--is at the core of this book. Terror, and how to confront it. “Understanding is a delaying tactic,” muses mission psychologist Johnson. “Only people who are afraid of the water want to understand it. Other people jump in and get wet.” So Crichton’s academic credentials may be impeccable--but only in letting them lapse does his researcher become a hero in this tale.

There are more than sonar echoes of Verne’s “20,000 Leagues Under the Sea” here--and that’s another charm of the novel. One of the subliminal ways Crichton achieves this is by titling every chapter (“The Monster,” “Beyond Pluto,” etc.)--a literary device that is rather out of fashion, and rather evocative of all those grand adventure yarns we read as kids. And, not incidentally, rather effective.

There are some problems with the book: Crichton’s dialogue tends to be a bit stilted at times, his characters a bit broad (there is the self-hating feminist; there is the weapon-mongering military man)--but these criticisms seem, at the end, pale in context, the context being that Crichton is a storyteller, and a damned good one.