I’m learning every day on Kurviger. Today: some topography. I didn’t learn in elementary school we had something like the Marianen trog in the North Sea. 
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Just linking the according Wikipedia article, it’s an extinct volcano 
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But -3300m ?? I think that’s ‘Doggersbank’ now. You can almost stand right there when the tide is out 
No idea, but Wikipedia lists the same number 