There had been many ship tunnels built in the UK a few hundred years ago, just before the invention of the railway. There were some surprisingly longes ones too

There had been many ship tunnels built in the UK a few hundred years ago, just before the invention of the railway. There were some surprisingly longes ones too: https://en.wikipedia.org/wiki/List_of_canal_tunnels_in_the_United_Kingdom https://en.wikipedia.org/wiki/List_of_canal_tunnels_in_the_U... Maybe the article is referring to tunnels on the sea?