The Star Dunes.
(Book)

Cruz, Sailor, Emmett, and the gang are on their way to Africa when Nebula misses their intended target and someone close to Cruz ends up on the brink of death. Secrets creep out from the shadows and leave Cruz with more questions than answers in this fourth title in the hit series.
A major discovery forces the Explorer Academy into the limelight in The Star Dunes, but Cruz has much more on his mind than 15 minutes of fame. A new face on board the ship brings Cruz's worlds colliding, just as a major close call tears them apart. En route to Africa, Team Cousteau is now down a major player, and Cruz can't help but be preoccupied by the hole in his life. The discovery of his mom's next clue leads him to the most exotic location yet—a vast desert—with no other information to lead the way, while an unlikely ally helps Cruz pursue another piece of the puzzle. Just as things seem like they might turn out alright, Dr. Fanchon Quills has a technological breakthrough which gives Cruz a glimpse into the past and reveals more about his future than he may really want to know.