Cassie's Coven: The Abalone Ring Mystery - Cassie's Coven, #4

Magic, Mystery - and Girl PowerWhen strange holes appear in the sand dunes around their small coastal town, and abalone start disappearing from the reef, Cassie's Coven will need to use science and their swimming lessons to solve the mystery. But as one of their own begins to act strangely, and a deadly spell is uncovered, Cassie discovers that being witches may pose as much danger, as it does fun, to her little coven.
This is a middle-grade academy series:Cassie's Coven: The case of the six-legged frogCassie's Coven: The bad apples mysteryCassie's Coven: The bye bye birdy mysteryCassie's Coven: The abalone ring mystery