[ncmncm]

Wondering if it was used for wartime production, for protection from bombings.

German aircraft production increased each month right up until substantial homeland territory was lost, apparently unaffected by massive bombing. Given its military futility, most of the bombing should probably be considered war crime. Air crews' lives were squandered.

[LargoLasskhyfv]

Unlikely. Parts of it are above the river bed, and stick out into the water. It's all rather fragile patchwork, not some massive reinforced concrete thing.

This sticking out is also the reason all the terminals for larger ships are downstream of it, because their keels would crunch it, even at high tide.

[nielsole]

When I was constructed, the tunnel did have 3m of sediment on top of it. This was required, as the tunnel was dug horizontally. Unless that changed over time, no part of the tunnel should be exposed.

[LargoLasskhyfv]

I do know that. But I think the graphics in Wikipedia visualizing the profile are wrong. I've been into exhibitions about it, which showed much more than that. Also it may have been different at different times because the river is dynamic, has been and is been dredged out, from time to time, as necessary.

The roof of the tunnel is too high in the water, to let contemporary large ships pass. Where large ships means the giant container freighters with 19.000+ TEU capacity. Mission impossible. Covered or not. This is not true for roll on/roll off ferries, or some cruise ships which seem similarily large, but aren't, at least not below the waterline. They just manage, but barely.

It is a barrier inhibiting ships with large draft/draught from passing. Which is why all the terminals servicing them are downstream.

[derda]

I dont know about the use as a shelter, but you might be interested in the fact that one of the largest german shipyards Blohm & Voss is right at the south side of the tunnel.

[1] - https://en.wikipedia.org/wiki/Blohm%2BVoss