Submarines surfacing up under ice always seemed like a wildly improbable thing to me.
Yah, if only a few feet thick... that's one thing. But ten or twenty feet... that stuff is as hard as concrete.
Something I learned about was the difference between driving through "new" soft ice and "old hard" ice. Experienced drivers could tell the difference by just looking at it. "New" in this context was ice less that just a few hundred years old. "Old hard" ice was ice that was many hundreds of years, to possibly thousands of years old.
Breaking "old hard ice" is sortof like breaking concrete of the same thickness, if only that concrete were floating on water in the Bering sea. It sounds exactly like that, from the inside of a steel ship, as well. :-)
I recall a LOT of the book being about searching for places where the ice was thin enough to surface...don't remember just how thin they said it had to be. Just remember it being tense, 'cause all the while I was thinking "get me OUT of here!" I would NOT do well at submarine duty.
Didn't know that about new and old ice. That's interesting!