I've always wondered why Rick Yancey called the second book in the 5th wave series The Infinite Sea.
The first one, The 5th Wave, and the last one, The Last Star, I understand; but if anyone knows I would be very interested to hear.
Yancey answered this question in an interview with USA Today (italics their emphasis, bold mine):
Q. Why did you decide to call the second book The Infinite Sea?
A. I really kill myself on titles, although The 5th Wave seems like an obvious title, doesn't it? You don't know how long that took me.
There's a section in the first book called "The Infinite Sea" and it's referencing that moment — which is one of my favorites in the first book — when Cassie has finally reached the safe room where somewhere in that room is Sam. She's wandering through it and seeing all of these kids looking up at her, and she imagines the walls of that room spreading to infinity filled with billions of faces of children. She calls it "an infinite sea of upturned faces."
I wouldn't call it a wave, but there's one particularly nasty thing that the Others have decided to do during this winter season after the first 4 1/2 waves have rolled out that involves children. The title harkens back to (Cassie's) feelings of "They are my responsibility now, not just my little brother."