Sphere is a science fiction psychological thriller, bordering on horror.  I first read it back in my late teens, and I definitely think that my reread benefited from the fact that I had forgotten large parts of the story, because this is a book that you're really best going into without much knowledge of what is going to happen. The one thing I did remember spoiled the book a bit - I think if I had reviewed this my first time through I might have given it a higher rating. 

I listened to the audiobook, and while I always enjoyed it while it was on, it took me a while to finish because it didn't hook me quite enough to compel me to come back to it. This changed around two thirds of the way in and I sped through the final third. I recommend this book if you like  underwater/isolated settings, atmosphere, thrills, not really knowing what is going on, and interesting science fiction ideas.