Last week, the world was riveted by the successful rescue of a youth soccer team as they and their coach were pulled out of a flooded cave in Thailand. The team had been stranded on a narrow rock shelf in the dark for two weeks, the way out blocked by turbid stormwater. The rescue involved far more than a few divers putting on gear and heading into the cave—it required a tremendous amount of technical skill and posed extreme danger.

But why, exactly, was it so dangerous? And what would it feel like to dive in those kinds of conditions?

I’m a professional diver with 16 years of dive experience, including safety diving and cave diving, and I have trained numerous scuba instructors. I also work full-time in a safety diving role, so answering the first question from a technical perspective is easy enough. The short answer is that all cave diving is dangerous (we'll dig into why below).

But to answer the second question, I decided to open my logbook and go back to a dive from many years ago—well before I was diving professionally. As a few select passages below highlight, this was a dive where things almost went fatally wrong.

Into the dark

My first experiences in cave diving were incredible. Once I slid under the surface and into a cave, I entered an alien world filled with incredible rock formations, strange sounds, and a darkness so complete as to defy imagination. My dive buddy and I had made several scuba dives in fully flooded caves and were truly enjoying our new-found sport. There was just one problem: while we were both experienced divers, with hundreds of logged dives under our belts, neither of us was actually certified to dive in a cave. And in a cave, things can go wrong—quickly.

Cave diving has five rules. These sum up the hard-won wisdom of the cave-diving community, as conducted through the analysis of cave-diving accidents and fatalities. Though the exact wording of each will differ from instructor to instructor, the rules are:

Be well-trained and do not dive beyond your certification level

Never use more than one third of your breathing gas to enter the cave—reserve one third for exiting and one third for emergencies

Maintain a physical guideline back to the cave entrance at all times

Never dive below the appropriate depth for your breathing gas mixture

Carry at least three lights per person—one main and two back-ups

Since these rules were introduced in the late 1970s (first as only three rules, later expanding to five), fatalities per number of dives have dropped among the cave-diving community. Today, the largest segment of fatalities in underwater caves comes not from certified cave divers but from divers not specifically trained by a professional cave instructor to be in that environment.

The first of these rules is therefore simple, and one that I broke badly: never dive beyond your certification level.

This rule alone would have made the rescue a challenge. The boys trapped in the Thai cave had no diving—and in some cases no swimming—experience when they had to dive into the waters blocking their passage to the surface of their dark prison. Their first experience underwater in a cave was probably terrifying, with very low visibility, buffeting from high rates of water flow, strange booming and rumbling sounds from their expelled breathing gas moving across the ceiling, and the glowing orbs of their rescuers' lights.

Get in line

On one dive soon after starting to cave dive, we were 100 feet (about 30 meters) into the cave and about 100 feet underwater—well beyond where natural light penetrated in that particular cave but not terribly far from the entrance. Suddenly, I noticed my buddy’s light beginning to move erratically. I was on one side of a restriction, a part of the cave that narrows such that only one diver can fit through at a time; my buddy was on the other side. I turned around and poked my head through to see what was going on.

One of the major technical challenges in the Thai cave was to get the boys through a 15-inch-wide (about 38cm) restriction in the cave that exited the water on an incline. The rescue divers reportedly stationed themselves in front of and behind each boy during the underwater portion of the rescue, which is standard practice in an emergency situation. Unfortunately, due to the small size of the restriction, the divers had to remove their tanks and push them ahead or drag them behind while also tending the victim and the victim’s scuba tank.

This would have been extremely claustrophobic for all involved (seriously—grab a tape measure, reel it out to 15 inches, and imagine squeezing yourself through an irregular hole about that wide at its widest point). For a while, all the boys would be able to see would be rock right in front of their faces while feeling rock scraping along their bodies on all sides. During their time in the restriction, the rescue divers would also lose some ability to check on the victim.

Breathe in, breathe out

After poking my head out of the restriction, I saw my buddy signaling that he was out of breathing gas. In that instant, the cave turned from a fun place to seek new things into a claustrophobic and confusing labyrinth. I rushed to my buddy and found that he had already switched to a tiny tank of breathing gas used for such emergencies (a so-called “pony bottle”). But the pony bottle, too, was reading low—we had forgotten to fill it before starting our dive. We spent a few moments assuring each other that everything was OK, then decided to return to the cave entrance. It was then that I noticed I was running low on air as well.

Another rule of cave diving is to use only one-third of your entire gas supply on the way into a cave—leaving another third to exit and the final third for handling emergencies. Unsurprisingly, this is called “The Rule of Thirds.” This approach also leaves enough gas to help a diver who has experienced an equipment failure breathe off a dive buddy’s gas tanks, making it back to the cave entrance even from the furthest point in the dive.

In Thailand, one of the first things needed after the soccer team was discovered was to deal with the logistics of supporting all of the dives required to effect a rescue. This would mean having support divers cache breathing gas tanks ahead of the main rescue divers so they would have enough breathing gas to make it to the boys’ perch in the cave and back out. Every time a dive was completed, the support divers would have to clear the used tanks and place new ones for the next set of dives, dragging tanks in and out of the cave repeatedly. Even these support dives were fraught with danger, as evidenced by the loss of one of the support divers who ran out of breathing gas on his way out of the cave after placing tanks for the main rescue divers.