Fathomless - Samantha San Miguel - Bog

The year is 1898, and Lulu Davenport is looking forward to summer vacation at her home on the southwest Florida coast. Her best friend, Algie Emsworth, will be spending his vacation with the Davenports, but his widowed mother has fallen on hard times, so he's gotten a job at the local grocery store. Lulu is looking forward to spending her days in the sun, but she hears about a possible haunting in a nearby abandoned fort and rumors of hidden treasure. Lulu along with Algie and her sister Frankie team up to investigate and discover some very real dastardly deeds going on, and the villain is much closer to home than they expected!