@ Sailor Steve: ...and now itīs a very embarrassing moment for me - I still thought that it meant how deep they were

Well, strangely enough the German title of Jules Verneīs novel is "20.000 Meilen (miles) unter dem Meer (under the sea) which is highly ambigious. I thought Jules Verne believed one could go that deep since he couldnīt have known that the deepest part of the sea is "only" about 14 km (which is less than 8 miles approx.)
You live, you learn....