Part of our series based around the work of Dom Robinson and his Deep Wreck Diver Youtube channel. Dom is doing his own form of local history detecting, with shipwrecks!
A recreational scuba diver can go to 40 metres. That is the limit, the point at which standard training ends and the physics of breathing compressed air begin to make the exercise genuinely dangerous. Below that, the rules, the equipment and the planning changes. Because now the risks have changed considerably.
The Lusitania lies at 90 metres.
Dom Robinson dives wrecks like her routinely. What follows is an attempt to explain what that means — not the romance of it, but the mechanics. What it takes to get down there, what happens to a human body at that depth, and why so few divers ever attempt it.
Why 40 metres is the ceiling
Scuba diving on air works because the body can manage the effects of breathing compressed gas at moderate depth. Below 40 metres those effects compound quickly.
Nitrogen narcosis (sometimes called “rapture of the deep”) is the primary concern. At depth, nitrogen behaves in the bloodstream like a mild anaesthetic. The effect varies between individuals, but the general pattern is predictable: impaired judgement, slowed reaction times, a loosening of the mental grip that keeps a diver disciplined and focused. At 30 metres the effects are manageable. At 60 metres on air, they can be disabling. At 90 metres, breathing standard compressed air is simply not compatible with arriving back at the surface.
The second hazard is oxygen toxicity. As depth increases, the partial pressure of oxygen in the breathing gas rises. Beyond certain thresholds, oxygen becomes toxic to the central nervous system: causing convulsions, underwater, with no warning. This is not a marginal risk. It is a predictable physical consequence of the wrong gas at the wrong depth.
The third is decompression. Spend time at depth and nitrogen dissolves into the body’s tissues. Ascend too quickly and it comes back out as bubbles — in the joints, in the spine, in the brain. Decompression sickness ranges from painful to permanently disabling or fatal.
The old answer, and why the new one is better
For decades, the technical diving community solved these problems with open-circuit trimix: a breathing gas blending helium with oxygen and nitrogen, carried in multiple cylinders for different stages of the dive. Helium does not cause narcosis. The oxygen fraction is reduced to manage toxicity at depth. On the ascent, the diver switches through progressively richer oxygen mixes at designated stops to clear nitrogen from the tissues. It works. Divers still use it.
But open-circuit trimix has limitations that become increasingly inconvenient at extreme depth. A diver burns through gas fast. Every exhaled breath is vented into the water as bubbles and lost. At 90 metres, gas consumption is high enough that carrying sufficient volume for a meaningful dive, and for the long decompression stops on the way back up, requires a considerable collection of cylinders. Helium is expensive. And the bottom time, even after all of it, remains short.
The shift that has transformed deep technical diving over the past two decades is the closed-circuit rebreather: the CCR. Dom uses one. Most serious deep wreck divers now do.
How a rebreather works
The principle is straightforward, even if the engineering is not.
In open-circuit diving, every breath is exhaled into the water and wasted. A rebreather recycles it. When the diver exhales, the gas passes through a scrubber canister packed with a chemical absorbent, typically a material called sofnolime, which strips out the carbon dioxide the body has produced. The remaining gas, now CO2-free, is returned to the breathing loop and inhaled again. A small amount of fresh oxygen is added automatically to replace what the body has consumed, maintaining the correct oxygen level throughout.
The result is a closed system. Nothing is vented. The diver is breathing a loop of gas that is continuously cleaned and topped up. The bubbles that trail behind every open-circuit diver, the immediate visual signature of scuba, simply do not exist. CCR divers ascend in silence.
The practical consequences of this are significant. Gas consumption drops dramatically. Where an open-circuit diver at 90 metres might exhaust their supply in minutes, a CCR diver uses only what their body metabolises: primarily oxygen, in modest quantities. A compact oxygen cylinder that would last a recreational diver a single shallow dive can sustain a CCR diver for hours. The decompression obligation is also reduced, because the rebreather continuously optimises the oxygen partial pressure throughout the dive, minimising inert gas loading in the tissues.
More bottom time, less gas, and shorter decompression. Those are the gains.
What the dive looks like
A CCR dive to 90 metres on a wreck like the Lusitania still requires careful planning and a structured ascent. But the profile looks different from an open-circuit dive, and the experience at the bottom is different too.
Dom has described the quality of time on a wreck with a rebreather. The extended bottom time that was simply not available on open circuit changes the nature of the dive. You are not racing the gas. You are not counting minutes against a cylinder pressure that is dropping fast. You can move slowly. You can look properly. A wreck like the Lusitania, at that depth, is too large and too complex to absorb in a twenty-minute sprint. The rebreather makes a more considered visit possible.
The ascent still demands patience. Decompression stops remain non-negotiable (the body still needs time to off-gas nitrogen safely) but they are typically shorter on CCR than on open circuit, because the rebreather has been managing the oxygen partial pressure throughout the dive rather than delivering a fixed gas mixture regardless of depth. The diver still hangs in open water, watching the depth gauge and the dive computer, waiting. But less of it.
The equipment and the commitment
A CCR is a sophisticated piece of life-support equipment. It is also, by the standards of the diving industry, expensive; new units used for serious technical diving cost anything from £7,000 to £12,000. They require meticulous maintenance. The scrubber canister must be repacked before each dive. The oxygen sensors that monitor the breathing loop must be checked and replaced on schedule. A failure of the scrubber — allowing carbon dioxide to build up in the loop — is one of the most serious emergencies in diving, partly because the symptoms can be subtle until they are not.
For this reason, every CCR diver carries bailout: open-circuit cylinders strapped to the body, ready to be switched to if the rebreather fails. The bailout is not optional. It is a mandatory redundancy, and planning its volume is a critical part of dive preparation. If something goes wrong at 90 metres, the bailout must contain enough gas to complete the decompression and reach the surface safely.
Training for CCR technical diving at this level is sequential and takes years. Open-circuit technical diving comes first: learning gas management, decompression theory, and the discipline that deep diving demands. CCR training builds on that foundation. The certification pathway is long, and it cannot be rushed, because at 90 metres the margin for error is narrow and the consequences of mistakes are immediate.
Dom Robinson has been diving for more than three decades. The technical certifications he holds represent years of accumulated training and thousands of dives. When he descends to a wreck at this depth, he is not taking a casual risk. He is operating within a framework that has been built carefully over a very long time.
Why do it
The question that non-divers almost always ask is the obvious one: why?
Why bear all this expense, complexity, training and risk to spend time at the bottom of a cold, dark sea looking at a corroded structure that most people would struggle to identify as a ship?
Technical divers answer this differently, but the answers converge. The Lusitania is still down there. She is still largely recognisable as a structure. She is still carrying her cargo and her history and the unanswered questions that have followed her since 1915. Most people will never see her. Most people don’t know she can be seen.
At 90 metres, in the quiet that a rebreather makes possible, the history stops being something you read about and becomes something you are physically present inside. The scale of the wreck is not an abstraction. The corrosion, the collapse, the particular way a century of submersion has transformed iron and steel — it is all immediate and particular and real.
That is the answer. The wreck is there. The technology now exists to reach it and stay long enough to understand something of what it holds.
Dom’s channel exists, in part, to bring that experience to the people who will never make the dive themselves. His remarkable videos are the next best thing.