After 15 minutes or so, I watched the needle pass half-way on the pressure gauge, and creep slowly but surely towards the red. As it passed serenely into my reserve supply, I signalled to Sophia – by turning the gauge towards her and tapping its face to draw attention to its near completed downward plunge.

Considering we were still skirting the seabed, she was surprisingly unconcerned. A planned slow ascent was required to avoid my lungs swelling tragicomically beyond their natural boundaries as the pressure maintaining their natural form reduced. The appropriate metaphor for the bad end of this cautionary tale is a bursting balloon.

Perhaps this unexpected responsibility for my own breathing further increased my intake of air. Either way, the consequences were soon apparent. As the needle ground to a final halt, I pondered philosophically how much emergency air remains in the tank when all measurable indications suggest there is none.

Does the life-giving mix noticeably dwindle – leaving you gasping nostalgically for what is no more? The answer was negative – and came to me abruptly. It was very simple. One moment, like those preceding it – you have air. The next – you don’t. Your lungs become instantly jobless. Time, more literally than at any other moment in my life, stood still. My mouth froze in a rictus of unemployed terror.

We can, like a whale, exist for a small period without the luxury of breathing, but the time constraints are reasonably limited. Just about enough time, it proved, to make a grab for the leg of the instructor who till then might have wondered if I wasn’t a little bit too old to be quite so clingy. Having now gained her full attention, the slit-throat hand signal I never expected to use, sprung readily to mind.

Within a few seconds, I had her spare regulator thrust into my mouth like a baby’s dummy. Entwined with this umbilical cord of rubber, we slowly corkscrewed to the surface without further mishap.

Back on the boat, all came clear. While my gauge was in the red, she had simply ‘misread’ my gauge – a simple enough error. I laughed along mirthlessly.