Deep specialty at Gam Ghi

Target depth thirty metres. Bottom time twenty-two minutes. Turnaround pressure one hundred and twenty bar. Safety stop at five metres for three minutes. Skills to run at depth: timed task on a slate, narcosis self-check using a memorised number string, neutral buoyancy hover for ninety seconds, then a controlled ascent to fifteen metres for the second skill set.

Mr. Huy briefed each skill twice. The first brief was the procedure. The second brief was the failure mode. What happens if you skip the narcosis check. What happens if you blow through the turnaround pressure. What happens if you let the ascent run faster than nine metres per minute because the surface is in sight.

I wrote the turnaround pressure on the slate before the descent. One hundred and twenty bar. The number had to be the first thing on the slate so it was the first thing I looked at when I checked anything else.

The descent was eight minutes. The wall at Gam Ghi falls cleanly, with a flat sand shelf at thirty-two metres and a row of soft corals along the edge.

Thirty metres feels different from twenty-one metres. The light is colder. The blue is heavier. Sound flattens. My breathing rate dropped without effort, which is part of what makes deep diving deceptive: the body becomes more efficient at the exact moment when small mistakes become more expensive.

I ran the slate task in two minutes, ten seconds slower than the same task on the surface. That two minutes ten was the narcosis tax. It was within range. I noted it on my slate.

The hover went cleanly. Ninety seconds of nothing, which is the skill the test is actually testing. I held a sand grain the size of a comma in my eye line and did not let it move up or down for the whole minute and a half.

At minute eighteen of bottom time, I checked my SPG and read one hundred and thirty bar. I had ten bar of margin until the turnaround pressure. I had four minutes of planned bottom time left. The current was light. The visibility was holding. There was a school of yellowfin barracuda hanging off the wall fifteen metres away, looking exactly like the photo every diver wants and has not taken.

I made eye contact with Mr. Huy and signalled the ascent at minute twenty, two minutes early.

The slip is the two minutes I almost stayed.

I did not stay. I went up. The procedure held. But the procedure held over a small private argument I had with myself in the four breaths between checking the SPG and signalling Mr. Huy. The argument was the same argument every diver has had in that situation, and it has a familiar grammar. There is margin. The conditions are good. The instructor is right here. The barracuda are right there. Two more minutes will not change anything.

The grammar is the slip. The fact that I went up does not erase the fact that I had to argue. The day the argument wins is the day the procedure is no longer holding the diver. The diver is holding the procedure, and the diver gets tired.

I am writing the slip down because the next time I make the same call at the same depth with a smaller margin and a more interesting fish, the four breaths of argument will be shorter. The procedure has to be the thing that takes me up. The barracuda do not get to take me up. Neither does my judgment of the conditions, which on a deep dive is exactly the judgment that the depth is degrading.

The turnaround pressure exists because of who I am at minute eighteen, not who I am at minute one. Minute-one me sets the rule. Minute-eighteen me would relax the rule if minute-one me had not made it expensive to relax. The rule is minute-one me protecting minute-eighteen me from a decision that minute-eighteen me is no longer the right person to make.

This is the whole problem of pre-commitment, on a wall at thirty metres, in clean form. The rule was set when I had no reason to want to violate it. It was tested when I had every reason to want to violate it. The test of the rule is not whether it was hard to write. The test is whether it survived the four breaths between the SPG and the ascent signal.