I took my bowie knife from the scabbard on my side, and, like the primitive warrior who must touch the body of a slain enemy, I tapped with the butt of the knife on the conning tower's rim.

Then one of the strangest occurrences of my life took place.

I felt a bone-numbing coldness in the water, where there had been none before; then a sound, a vibration, like a wire cable snapping, rang through the entire length of the submarine. The conning tower began to right itself in the current, the metal plates on the hull grating on the sand, and clouds of silt and trapped oil rose from under the keel. I watched in horror as the submarine seemed to poise itself just above the gulf's floor, streamers of moss fanning back from the tower like tattered battle flags, then dip its bow downward into the darkness and slide over the edge of the continental shelf, my bowie knife toppling onto the rising stern, while sand sharks spun like minnows in the wake of its screws.

I discovered later that there was no mystery about the U-boat. It had been caught recharging its batteries on the surface, shelled by a U.S. Navy destroyer, then blown out of the water by depth charges, its spine broken; since that time it had scudded and bounced with the currents up and down the floor of the gulf along the Louisiana coast.

But sometimes in a dark moment I wondered about the crew that had gone down in a scream of sirens and whistles and torrents of water bursting through the ruptured plates or cascading down the tower that no one could close in time. Did they claw one another off the ladders? Were they willing to blind or maim or kill one another just to breathe air for a few more seconds? Did they regret embracing the scheme that would make the lights go out all over the world?

Or were they still sailing beneath the waves, their skins pickled in salt, their uniforms nests for moray eels, their plan to turn the earth into a place of concertina wire and guard towers still on track, as certain in prospect as the phosphorescent and boiling wake of a torpedo streaking toward a distant ship silhouetted against an autumnal moon?