A word you might use for scuba divers is adventuresome. A word that might describe scuba divers who venture into underwater caves might be daredevil.

But what do you call scuba divers who take the greatest risks of all and dive to explore hazardous shipwrecks that are so deep as to challenge human physiology? Crazy seems an apt word, but Robert Kurson prefers shadow divers.

"Deep shipwreck diving is among the world's most dangerous sports," he writes in the electrically charged adventure tale Shadow Divers. "Few other endeavors exist in which nature, biology, equipment, instinct and object conspire - without warning and from all directions - to so completely attack a man's mind and disassemble his spirit. Many dead divers have been found inside shipwrecks with more than enough air remaining to have made it to the surface. It is not that they chose to die, but rather that they could no longer figure out how to live."

Often, wreck divers' greatest threat is themselves. At extreme depths, the body's chemistry is thrown out of whack, creating a sometimes deadly, often judgment-impairing condition known as narcosis. Add to that risk the fact that the slightest equipment malfunction is greatly magnified at the limit of a diver's descent (about 200 feet).

The wrecks themselves are confusing labyrinths of entangling wires and shifting debris. And on top of it all, the slightest movement can stir up silt and eliminate all visibility save for murky shadows - giving those who explore wrecks the name "shadow divers."

Shadow Divers has as its main characters two divers - John Chatterton and Richie Kohler - who, in 1991, discovered off the coast of New Jersey one of the most prized finds in the history of diving: a World War II German U-boat.

About 65 U-boats disappeared during the war in unsearchable water to become unfindable graves. The discovery of this U-boat comes fairly early in the story, which then backtracks to give us the stories of Chatterton and Kohler. Author Kurson has an admirable ability to tell different branches of his story deftly and to weave them in such a way as to enhance rather than diminish the pace of his book.

But while the discovery of the U-boat might seem like the fitting climax to this story, it's not. The search Chatterton and Kohler undertake to discover the identity of the U-boat and then the stories of the men who manned it is as interesting and exciting as the most heart-pounding chapters on the underwater exploration.

For them, the payoff is not the sought-after relics they might salvage or even the fame of making such a significant discovery. Instead, it's the satisfaction of learning how and why the U-boat got where it did and who took it there.

In that way, Chatterton and Kohler paid a debt to others who lived and died beneath the waves.

Shadow Divers has all the thrill of discovery as a favorite of several years ago, Ship of Gold in the Deep Blue Sea. But what ultimately sets it apart and makes it such a great read is not the discovery itself, but what inspires those who seek to discover.

Dan Danbom is a freelance writer living in Denver.