John Hopkins

And if he is having to focus on himself, it means he cannot focus on Dave.

He cannot go any deeper.

The bleak situation leads to a painful decision.

With one last glance of the unmoving light of his friend, John begins to head back to the surface.

Having gone deeper than he planned, he now needs to work out a new schedule for his ascent to limit his chance of getting the bends.

Also known as decompression sickness, this occurs when a diver resurfaces too quickly, not allowing enough time for nitrogen, another gas absorbed into the body through breathing, to be safely released.

Without proper decompression stops, the nitrogen can form highly dangerous bubbles in the blood and tissues, leading to dizziness, difficulty breathing, and even paralysis.

Above Don right now is almost 250 meters of water, over 800 feet.

It's going to be a long, dangerous journey.

On the surface, everything is running smoothly.

Nobody has any idea of what has just happened in the depths of the sinkhole.

As planned, the next support divers enter the water and descend to 150 meters, where they are scheduled to meet Don.

But when they get there, he's nowhere to be seen.

Just before they have to leave, the two support divers see a single light rising up through the darkness.

At this distance, they have no idea whether it belongs to Dave or Don, but its significance is clear.

Only one of them is coming back.

The divers retreat to the surface, to break the news to the rest of the team.

Meanwhile, Don continues his slow, steady ascent, all the time looking back down to see if there is any movement from Dave's light, clinging to the unlikely hope that his friend might still be alive.

Eventually he reaches 150 meters, where a group of emergency tanks are clipped.

With the gas in his own cylinders now depleted after his rebreather malfunction, he's reached them at a crucial time.