The Sherwood beeped. Time to go up to the next stop. 80 feet for seven minutes. Tusker checked his pressure gauge. He’d breathe down the big tanks as long as he could but soon they’d need something with a little more oxygen to speed decompression. He glanced over at Sam’s gauges, upside down. She was fine. He found himself studying her as they hung in the current. She floated horizontal and motionless, her fins out behind her, perfect form. Decompression was best if your entire body was in the same plane, to allow equal off-gassing from all tissues. Even here, behind the tangle of hoses and tanks, she looked athletic and confident.

Beep. Time to ascend. At 60 feet, Tusker and Sam switched to their 80-cubic-foot bottles of nitrox, a blend of 50 % oxygen and 50 % nitrogen. The nitrogen would be nearly harmless at this depth and the extra oxygen would accelerate their decompression. These tanks would have to take them up to the ten foot stop, where they’d finally switch to a small tank of 100 percent oxygen. Tusker tried to breathe evenly. He had to make this tank last close to three hours.

And so they went on, being pulled south at progressively shallower depths. Tusker was shivering noticeably now and Sam rubbed his arms. They were up to 30 feet and the water was warm now, but the chill had gotten into his core. He was also dehydrated from sucking dry gas for three hours. His tank was down to 700 pounds per square inch, less than a quarter of its contents remaining. It would be a close call. He hoped he didn’t need to ask Sam for any of her gas.

At one point, the baritone gurgle of a boat motor seemed to come close and they both swiveled their heads, looking for it. It sounded too big to be Roland in the skiff and Tusker’s hope faded when he realized there was nothing he could do anyway to signal it. Their only hope would be if someone saw their bubbles. But at dusk, in rolling ocean swells, this was unlikely. He wished he’d brought his reel. He could have hooked on his surface marker buoy to a line and sent it up with a puff of air to drift along with them, as some signal of their presence. Rookie mistake, not bringing it. He’d been in such a hurry to clandestinely load the skiff earlier that he’d forgotten it.