Q&A: Why Brest's U-boat pens remain

Did the U-boat pens survive because demolishing them required too much explosive?

That is the explanation accepted in this brief exchange, with French demolition attempts mentioned but not described in detail.

Surviving U-boat pens are identified in the view of modern Brest. A viewer attributes their survival to the amount of explosives demolition would require, and the reply broadly agrees. French attempts to remove them are mentioned without a detailed demolition history.