When redheaded Judy Bolton and her friend, Holly Potter, set out after the green car, they never dream that the chase is going to lead them into a far greater mystery than just the theft of Holly's typewriter.

What Judy sees under the surface of the pond by the beaver dam is enough to mystify—and terrify—anyone, and the young detective can't be blamed for not immediately realising the significance of the "apparition." (Goodreads)