Big and powerful sharks, but a bit dumb.(Knight Ridder Newspapers)

Byline: Susan Cocking

COCOS ISLAND, Costa Rica _ Lying 100 feet deep on a rocky tabletop seamount, I am surrounded by scores of scalloped hammerhead sharks. Lazily, the sharks _ some up to 10 feet long _ circle the pinnacle, occasionally tilting an outrigger-mounted eye down to glance at me.

Resting motionless on the rock about five feet from me is a small whitetip reef shark. We, too, exchange glances, and I notice that this four-footer bears a substantial bite wound in its flank. The wound has begun to heal over, but it is deep enough that I can see distinctly the dental impression of whatever bit it. These were some very big teeth.

''Yeah, you better lay low,'' the whitetip's war-weary look seems to say.

I look up again to see a hammerhead about 10 feet above me, and I expel a somewhat anxious breath into my …