My heart beats louder than the sound of my breathing as I descend near a scalloped hammerhead cleaning station off Cocos Island, Costa Rica. Without bubbles, all my senses seem more acute and heightened. I am aware of the mechanical aspect of my breathing and the drum of my pulse in my head, but these human signals are not passed through the water — fish barely acknowledge my presence. They shift enough to allow my passage, as if I am just another ocean denizen.

I see hammerheads in the distance, cruising deeper, slowly sweeping their signature cephalofoil back and forth. Then, from the corner of my eye, I see a single hammerhead gliding over the rocky reef where I am kneeling. My black wetsuit blends with the dark rock, so I hope only motion could give me away. The shark is still swimming — almost at me, but slowing as the butterflyfish start to ascend to their salon guest. I realize the shark appears to be in some kind of stasis — not really swimming, but gliding forward like a moving boat with the ­motor turned off.

Even in my crouched position, it’s clear I’m in for a near-miss — at that moment, the tip of its left ­pectoral fin nicks my dome and triggers 6 feet of muscle and cartilage into an explosive exit. My heart beats its ­usual rhythm once again as a grin wraps around my mouthpiece.

Yes, perhaps a closer encounter than intended, but rebreathers are one more way to minimize our presence in the submerged world — giving us a better chance of seeing what the ­bubble-blowers cannot.