BOOK REVIEW : When Human Nature Is as Dark as a Cave : BLUE CRYSTAL, <i> by Philip Lee Williams</i> , Grove Press, $19.95, 288 pages

This is about caves and those who go into them and why they do. It’s like heaven down there, they say, or hell, or both.

It is both for Sam Preston, who owns a hardware store in Sheppard, Ky., but spends most of his time in Blue Crystal Cave. The grotto was named by Sam’s grandfather, who capitalized on local lore holding that beneath the town was a magnificent blue crystal embedded in limestone at the end of a lost chamber.

Thinking tourist , Grandpa sent to South Africa for a 100-pound amethyst, rigged up lights, made improvements in the cave system and counted his money.

Sam’s father, Allen, inherited the cash cow and ran it until 1958, when part of the cave collapsed, fatally trapping 17 touring schoolchildren. A year later, Allen committed suicide.

Sam’s ongoing obsession with the cave has cost him his wife (unwilling to play second fiddle to a hole) and an eye, lost when he tried to blast open another entrance to Blue Crystal. Neither loss has diminished his fascination. “There’s a cathedral down there,” he tells Mary Beth, a new girlfriend. “This is where God’s marrow begins.”

On the surface, Sheppard, where Mary Beth owns a bookstore, is a tidy town, pleasant, friendly, “a few cars passing through, almost apologetic for disturbing the early-morning peace.” We meet its citizens informally, as if running into them on the sidewalk: Sheriff Tom Meade, solid and competent; Mayor Douglas Dickens, a bit of a jerk but tolerated until the next election; mechanic Mose Johnson; store owner Jimmy Jones, whose “slow-motion life calms everything around him. . . .”

We are just getting to know this placid town, to like it, when author Philip Lee Williams abruptly introduces a pretty fair working version of evil incarnate. Hurtling toward town in a stolen car are five surpassingly vicious young layabouts bent on stealing the legendary blue crystal or the Prestons’ reputed fortune, whichever comes first.

Driver Hermie is a drooling cretin looking for the meaning of life, i.e. “What do they do with your blood when you die?” Christiann and Misty are along for the ride, the thrills, the sex. Armed-robber Clay has shared a prison cell with Bobby, who was jailed for killing a man with a tire iron.

Subtext is the cave itself, sometimes benign, more often a brooding recess of the mind as well as of the Earth. The sheriff loathes the cave; his only son had been entombed in its collapse. Bobby fears it; his father had coughed his life out in a coal mine.

Above ground, as Bobby’s squalid cohort corners Sam and Mary Beth in Sam’s cabin, a symbolic torrent blows in from Indiana. Sheppard is threatened by the rising Coollawassee River. Sheriff Meade organizes the townsfolk into a sandbag brigade. At the same time time, Sam, his face battered to jam and now blind in his good eye, escapes, slithering through muck and down his private sinkhole into Blue Crystal Cave, which he can read like Braille. Greed prevailing over dread, his tormentors follow him into the black.

It is a classic contrast: high-minded heroics above, subterranean impulses below. Good and evil. Ego and id. And a cracking good tale while it lasts.

Unhappily, the story is drawn down into a Stygian struggle that spins out of control: guns, knives, bodies bobbing in the flooding grotto. Blind salamanders. And bats. Bats in Christiann’s hair. In Hermie’s dead mouth. Bats truly out of hell.

Williams has taken a perfectly serviceable story and Stephen Kinged the heck out of it, trying to top himself at every twist of the tunnel. Perils-of-Pauline rescues. Superhuman feats of strength. Eventually, it’s just plain silly.

Silly--but creepy all the same.