The Promise

Some Mysteries are Better Left Unsolved
Back from a harrowing adventure on the Arctic ice, most people would need a rest. But young adventurers Peter, Catia, and David aren't most people. A centuries old mystery is sitting in the corner of the underground cavern waiting to be solved. The sphere has been gathering dust for longer than any of the oldest caretakers can remember. It's not till a nasty disgruntled caretaker named Luke teams up with the young teens that a solution is possible. But Luke's selfishness raises the stakes for everyone.
The third book in "The Promise" series. Check out book one, The Promise: A Perilous Journey and book two, The Promise: Discovering Their Gifts.