A line of patch reefs surrounded meadows of sea grass in the shallows of Dinah's Land, a dive site on the north side of Milne Bay in Papua New Guinea. The little reefs silhouetted against the fading light looked like miniature ruins of medieval castles. This was a small but busy kingdom: Each coral head was a cleaning station, home to blue-striped cleaner wrasse and several species of cleaner shrimp.

Our dive boat, the Telita, was anchored off a curving beach. Its owners, Bob and Dinah Halstead, are remarkable explorers, photographers and naturalists whose exploration of Papua waters has unveiled some of the most pristine reefs in the world.

I discovered that diving in Papua New Guinea demands strict adherence to protocol: Wake up, get in the water and do not get out except for film or air. Every day curious kids from the community paddled in their outriggers out to the Telita to hover over our bubbles. My outrigger guy always left early, which made me think that watching a photographer shooting nearly invisible creatures must have been like watching an accountant at his desk or NASCAR with the volume off.

At the end of a long diving day, in the last twilight, I swam by a cleaning station under a brain coral head. I could just make out a tiny white moray eel with what looked like a beautifully applied henna-tattoo mask; it was a grey moray. The eel was being cleaned by a transparent cleaner shrimp that looked more like a big mosquito than a shrimp. In the diminishing light I focused on what I could see, which was the little eel's black eye. I positioned the strobes and began to shoot up the last roll in the last light - true last-gasp photography.

At home a month and a half later I saw the roll again, this time on my light box. There in my loupe, lurking behind my tiny speckled grey moray was a surprise: a big spotted moray looming gigantic in the shadows. This captivating moment was caught completely by accident - a rare but great joy of underwater photography.