John Hopkins

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.

It's an unrealistic shred of hope, but hard to shake.

On the surface, dive marshal Werner has set an emergency plan into motion, and tension mounts as people wait to discover which diver is coming back.

The question is soon answered.

When Don reaches 80 meters, another support diver drops level with him.

Immediately the diver sees Don's wrecked rebreather controller.

It's obvious he has gone deeper than planned, and that can only mean there was a problem.