In article .com,
lonelytraveller writes
What I meant was, wasn't it possibly for them just to add the
prefabricated steel tunnel rings into an already existing cavern? It
would save a lot of effort.

Even if such a cavern existed (which I doubt), putting a line of steel
rings across it isn't easy - they'll all fall down! You need to build a
viaduct across it (as was done on line 7bis in Paris).

Or are you suggesting that there's a natural tunnel-shaped cavern under
East London?

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