I knew I was descending. I could tell by the pressure in my ears and by the knowledge that I was substantially over-weighted. I also knew that the bottom was at a depth of 40 feet or so. I'd land soon, but when? I tried my light in the inky darkness but found that it could not even illuminate my feet, which I presumed were still below me. Disturbed by the feeble orange glow produced by my overmatched light, I switched it off and continued my disorienting fall into the black void.The end came suddenly, and I collapsed upon the hard clay bottom like a marionette whose strings had been unexpectedly severed knees and elbows akimbo. The force of the current became apparent then, and I struggled briefly to orient myself to face it. I turned on my light and stabbed my loaned screwdriver into the hard clay marl. And there I was - pegged securely to the bottom of the Cooper River near Charleston, South Carolina.Bordered intermittently by wire grass marshes and stands of bald cypress, the scenic waterway cuts through the fossil-rich Hawthorne Formation in South Carolina. All dives in the river must be timed to the tides and range in depth from 20-55 feet. Currents run from moderately wild to reg-purging, head-vibrating madness. I had come to the river in search of prehistoric shark teeth, specifically those of the 60-foot long charcharocles megalodon that lived during the Miocene-Pliocene epochs, or 6-10 million years ago. My light illuminated the bottom some two feet in front of me, and I could see sediment racing through the light beam in the current, as well as a few skittering shrimp and a monstrous blue crab. What chance did I have of finding a tooth that had been lost for millions of years in this primordial pea soup?After rooting around for several minutes, I spotted what appeared to be the side of a tooth in a sandy depression. I dug a bit, pulled a bit and saw a huge tooth emerging. My heart raced until it came loose and I noticed that a third of the tooth was broken off and missing. Even so, the remaining fragment covered the palm of my hand. This was certainly no museum piece, but I felt the indescribable thrill of finding treasure. My mission had been accomplished.The bottom of the Cooper River is littered not only with megalodon teeth, but also with the teeth from many other types of prehistoric sharks. During a full day of diving, my finds included a finely serrated tooth from megalodon's predecessor, charcharocles angustidens, as well as a fine tooth from an extinct mako. To arrange your own dive into the distant past, contact Captain Tom McMillan at (843) 693-8485. For more information on these fossils, see Steven Alter's excellent Web page at www.megalodonteeth.com.