Diving. Why descents go straight to the bottom: the hidden logic behind a fundamental safety rule

The Introvert Traveler
Nov 29
3 min read

The image above shows the profile of a dive I did last August in the Red Sea. Everything went fine, but things could easily have taken a bad turn…

In our group there was a Chinese guy who was completely unaware of basic diving rules—reckless, essentially insane. On paper he had all the required certifications, but if he ever took the courses, he clearly wasn’t paying much attention. He would shoot from –40 metres to –10 and then back down to –40 in a matter of seconds with alarming nonchalance, and that wasn’t even the worst of it.

During a dive at Daedalus Reef, where everything had gone smoothly, just as we were about to start our safety stop he saw some gigantic trevallies down at 40 metres and, mistaking them for something else (tuna, sharks, who knows), he plunged down to film them with his damn GoPro.

My wife—who takes the “stay with your group” rule a little too literally—followed him. The problem is that in doing so they both drifted too far from the wall, where there was no current, and ended up in open water, where the current was very strong. Unwilling to become a widower, I darted after my wife to politely reprimand her (it took some creativity to translate “to hell with the Chinese guy, he keeps putting all of us in danger” into underwater sign language) and bring her back toward the boat’s mooring line which, against the current, felt more and more unreachable—while my wife was down to about 30 bar.

So what’s the issue with the dive profile? That spike at the 45th minute, dropping to –26 metres after the profile had already risen steadily to 10 metres.

The mistake obviously wasn’t big enough to cause problems for me or my wife, but it would’ve been better to avoid it. It’s worth noting, however, that once we hit 26 metres, the ascent along the line was cautious and slow, fully respecting the computer’s guidance.

What happens in your body when you descend

When we learn to dive, one of the first safety rules we’re taught sounds almost carved in stone: reach your maximum depth first, then ascend gradually, without going deeper again.

Many divers follow the rule, few truly understand why it exists. Yet behind that simple instruction lies all the physiology of gas absorption, the logic of decompression models, and decades of accumulated experience.

In this post, I’ll take you behind the scenes of decompression theory to explain why this rule exists, and why it still applies during repetitive dives.

The body and inert gases: what happens when we descend

As you go down, pressure increases and the nitrogen in your breathing mix dissolves progressively into your tissues. Some tissues absorb quickly, others slowly—that’s why decompression models use “compartments” with different half-times.

This dynamic works well if the dive profile is linear:descent to max depth → bottom time → gradual ascent.

It’s the scenario tables and dive computers were designed for.Changing the order of depths alters the logic of the model.

Why reverse profiles should be avoided

A reverse profile (going deeper again after having already ascended) introduces two problems:

Your tissues are already partially saturated. A second descent increases the nitrogen load in an unpredictable way.
The decompression model isn’t built to handle this scenario efficiently.

Within certain limits, reverse profiles aren’t necessarily dangerous in recreational diving. But a key point remains: they’re best avoided because most of the safety data we rely on comes from dives with the traditional “deep → shallow” structure.

Multilevel dives: why they only work in one direction

Multilevel dives became popular with computers and with tables such as The Wheel or the eRDPML. But the logic is always the same: deeper level first, then progressively shallower ones.

Tables, for example, don’t even allow you to descend to a deeper level again. Computers can calculate it, but that doesn’t mean it’s a good idea—the model is optimized for the standard profile, not for improvisations.

And what about repetitive dives? The rule still stands

When doing multiple dives in the same day, residual nitrogen becomes significant. Older U.S. Navy tables penalized a second dive deeper than the first, because tissues were already saturated to some extent.

The principle still applies today: the first dive should be the deepest; the later ones progressively shallower.

This minimizes decompression stress and aligns best with how tissues load and off-gas nitrogen.

In summary

The correct dive sequence—deeper first, then a gradual ascent with no return to depth—isn’t tradition, but physiology. It simplifies planning, stabilizes decompression models, and reduces risk.

Next time you dive, remember: it’s not just routine. It’s one of the pillars of diving safety.