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None of this is relevant because the cave contains a restriction so tight that a rigid pod wouldn't fit through. The last thing they want is a solid object wedged into the restriction.

The SpaceX design is apparently flexible to fit through the cave, so some buoyancy adjustments would be necessary.

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With a flexible pod, you'd partially deflate it to get it through awkward areas, which would make it negatively buoyant. Once re-inflated to the previous pressure it would be neutrally buoyant once again.

Re-inflation could be done via the same gas cylinder that's being used to replenish the air in the pod.

But if it's flexible then, like a driver with a BCD, it won't remain neutrally buoyant at every depth.

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