The Kraken Wakes by John Wyndham
The things that scared us as children tend to stick with us all our lives; just ask the good people at Kindertrauma. Many adults retain a fascination for things such as the Daleks and Pennywise the clown, to name two common examples.
But there are other, more subtle terrors. I read The Kraken Wakes when I was about 12 (in a lovely 1950’s penguin edition, too), and didn’t think it was too scary. Aliens, end of the world- I’d heard it all many times before. In this case, the unseen creatures cause mankind trouble on a scale that is simply geographic- they melt the ice-caps and flood the world.