How Zzzap sees submarines

His gaze swung back to the ocean. He’d been searching all the currents and patterns for another boat, looking for one of them to stand out like a copper penny in a pile of silver dimes. He changed his focus.
He looked for the quarter in the pile of dimes.
A sub was going to be subtle. It’d be insulated to keep in heat so nobody froze. And it’d be shielded from radiation. No point having a nuclear sub loaded with missiles if the enemy could just find you with a Geiger counter. Plus the crew wouldn’t have anywhere to go, so all possible radiation sources would be shielded for them, too.
Just ahead of him and off to his left, in the middle of the spraying lines and spinning circles, there was a dull spot. Just enough to register. Long and thin, like a gigantic cigar. It was about a mile past the island and picking up speed.

Excerpt From Ex-Isle, pg 630-631
Peter Clines
https://books.apple.com/us/book/ex-isle/id989619017