Burdened with four tanks, a ­rebreather and a load of camera equipment, I swam up the steep wall from a 450-foot-deep dive with Brian Kakuk, my ­longtime exploration partner. Expansive orange sponges adorned a cliff punctuated by lacy outcroppings of hard purple coral. Brian slowly spooled his reel, which led to a weighted line swinging like a pendulum over the Challenger seamount. Arriving at its summit, we were still 220 feet deep, and a long way from completing our dive. Ahead lay several hours of decompression in the infinite blue.

We had joked with our NOAA ­project ­director about diving in the Bermuda ­Triangle — we had serious concerns about large animals that could rise out of the deep. This is the favorite territory of giant squid, and we would be exposed and vulnerable. During our prep, a hard rule had been agreed on: Once we reached the line hanging from the boat, no matter the circumstance, we would not leave it. Losing track of our vessel could result in being lost in the middle of the Atlantic Ocean.

As Brian and I slowly worked our way upward, I could see the silhouettes of our teammates backlit by the sun. Everything was on track. We just had another couple of hours before we could safely surface.

Suddenly a large dark form started to take shape in the distance. My heart rate soared as my mind tried to interpret what I was seeing. Soon, alarm melted to wonder as the winged shape became an enormous manta ray, swimming straight for us. I powered up my camera just as it moved away from us.

Glancing toward my ordinarily safety-conscious partner, I saw fascination and excitement in his eyes. Uncharacteristically breaking protocol, Brian left the line, shouting a muffled message through his breathing loop. Like Michelangelo’s Adam on the ceiling of the Sistine Chapel, he ­extended a hand, almost touching the ray with his outstretched finger. Calling out to the graceful fish, he said, “Take me to your leader!” Fortunately, his excursion was short, and he returned safely to the line to share a hearty high-five. We might have bent the rules a bit, but it sure made a long deco pass quickly.