To get an idea of what it’s like inside an underwater submersible, we come to an industrial estate in the Dutch town of Breda.

This is the home of U-Boat Worx, which makes a range of submersibles.

Most of them are bought by private yacht owners to use for their own projects, but plenty are used for research and exploration.

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None of them go as deep as the wreckage of the Titanic; they’re not designed for those depths.

But in front of us is the Cruise Sub-7, a seven-person submersible that’s got roughly the same dimensions as the missing Titan vessel.

One difference is immediately obvious. At the front and back of this machine is a large acrylic window that allows a wide view of the outside world.

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The Titan is different, with a very limited view through a single porthole.

I climb a flight of steps to reach the top of the submersible, then go down four steps of a ladder into the sub itself.

Three things strike you straight away – the light that comes in through the windows; the sophisticated instrument panel and the small size.

The light will have gone even by the time you get to a depth of 300m (980ft) – it’s dark at that depth.

But the sub has powerful exterior lights and that would ease the sense of claustrophobia.

On the Titan, you don’t have that – it must feel much more as if you are trapped in a box.

The instrument panel is another key difference – the Titan, as everyone now knows, is partly manipulated by an old Playstation controller, and does not have international safety certification.

The Sub-7 is fully certified, and has an array of controls for normal manoeuvres, emergency response and communications with the surface.

All of that provides reassurance. The list of “fail-safe guarantees” in case of problems is long.

But then there’s the size.

You’d only expect to stay on the Sub-7 for an hour or two – eight hours at the most – and I don’t think you’d want to stay longer.

With seven people inside, it would feel cramped.

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There is certainly not enough space to stand up straight and moving around is difficult.

The prospect of being trapped inside a submersible of this size for days on end would be utterly grim.

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