Dan and I were last into the water, slipping down the buoy line on the Green Rock dive site.

Divemasters often work as dive guides, navigating paying customers around dive sites and pointing out interesting sea life. We'd been learning about this in our course, but this was the first time that Dan and I were going to try it out, on each other.

We went down slowly, feet first, looking down, checking the visibility and feeling our bubbles caress our cheeks.

The line was tied to a big granite boulder, a good reference point for us to find our way back to it. We checked our compasses and started swimming south when suddenly I inhaled so sharply my mouth hurt. I grabbed Dan's arm and, made speechless by equipment, pointed frantically at the rocks beneath us.

Dan turned to face me in alarm. His thumb and forefinger questioned me, looped in the OK sign.

I put my right palm over the back of my left hand and wiggled my thumbs enthusiastically—the dive sign for Koh Tao's namesake animal, the turtle.

This tao sat about 15 feet beneath us on the coral-covered boulders, chewing his lunch and completely unconcerned that we were there. Koh Tao might be named after turtles, but actually seeing one is not very common—I'd seen one the week earlier, and Dan had spotted one while snorkeling in December Each sighting is a cause for a lot of thigh-slapping excitement and jealousy from other divers.

So, all our plans of mentally mapping the dive site disappeared with our bubbles and we hovered closer to the turtle, just watching. Turtles eat coral, and lots of it. They eat in the sea like their landlocked cousins do—messily. For every chomp of its beak, a half-mouthful wafted slowly downward. Cautious parrot-fish darted in to catch the remainders before they settled on the boulders.

The turtle caught sight of me and hesitated a moment, a great yellow eye rolling in the socket. I kept still in the water, inhaling slowly so the bubbles from my exhalation wouldn't worry it. I looked harmless enough, I guess, because it continued eating the leafy soft coral.

After fifteen minutes we decided to swim away and try to circle the dive site, as we'd planned. We spotted nudibranchs, angelfish and anemone fish, but nothing as extraordinary as the turtle. Thirty minutes later we made it back to the buoy line and found it still lunching. We spent another five minutes with the turtle until our air supplies got low, and then we reluctantly headed surface-ward, contemplating the turtles of Koh Tao.