Call Me Z

The cellar door is locked for a reason. Z is going to find out what it is.
 
It is the summer of 1978, and Z White has one week at her grandparents' farm in Summerville, Georgia before real life starts crowding back in. She plans to ride along on Granddaddy's egg deliveries and fill her purple notebook with anything strange enough to investigate. She already has one mystery on her mind: why has everyone called her Z for as long as she can remember? Then she finds out Theo is staying too, and suddenly her quiet week is looking a lot less quiet.
 
Everything changes when a thunderstorm sends the family racing into the old cellar beneath the farmhouse. One wrong step lands Z in a hidden room lined with mason jars full of clear liquid. Granddaddy gets her and Theo out fast, but he does not explain what they saw. Soon the room is locked.
 
Z knows when something does not add up. More clues begin appearing around the farm, and Theo turns out to be a better partner than she ever expected. With only a few days left before they are sent home, the two of them start pushing closer to a secret the adults clearly do not want discussed.
 
What is hidden behind that locked door, and why is Granddaddy so determined to keep it there?