There's a children's adventure novel that I read in elementary school and despite remembering all the important bits of the plot, I've never been able to locate it again. I read it in probably the late '80s, but it was a fairly old book at the time, so I suspect it was actually written in the 1960s, probably contemporaneous with the story's setting.
The book is set in the 1950s or '60s at a private girls' school on a tiny island just off the coast of wherever they were (England, probably, but it could have been San Francisco) connected by a small bridge to the mainland. The story starts when almost everyone is away except for the main character (a young girl) and a few adults (two, I think), and the island abruptly turns and starts to drift away, tearing the bridge apart as it goes. I think most of the story is just the strange adventures they have on an island that has abruptly set sail.
I think I remember a bit where they're discussing their good luck that the school was there, because the kitchen is stocked for dozens of girls, which means feeding three people on it should last a good long time.
Eventually they find their way underground, and it turns out the island was a ship or submarine all along, a secret weapon from World War II meant to trigger if the homeland fell, and it's packed with an enormous bomb. The soldier piloting it has no idea that WWII ended, as his radio failed in the middle of the war, and his orders were for absolute secrecy. Something the characters did triggered his only remaining communication device, which was the 'launch' command, so he has set sail for some important enemy city (in Japan, I'm almost certain) and is fully intending to blow up the whole thing when he gets there. I think he heard them building the school above his head, but he misinterpreted the construction noises as bombs falling, or something like that.
I'm almost sure the book's cover was orange with yellow markings, but that might not help much; it wasn't a proper book cover, it was one of those super-durable covers they used to put on library books that has the name on the spine but otherwise doesn't really have anything to do with the contents.