I.D. this children's book?

I faintly remember a book I read when I was younger. It was about a new kid in a small town who sets out to solve the local mystery. Standard fare, except for what the mystery is: a pond where things vanish. Ducks, dogs…I think even people. It was very spooky, but I misremember what actually caused the disappearing.

Was it a huge turtle?

And did anyone else read this? What did you think of it?