The Legend of the Rift (Seven Wonders #5) by Peter Lerangis

This is the epic final installment of the series. This edition begins right after Aly has been kidnapped by King Uhla'ar and pulled back into Atlantis. The rift is guarded by a behemoth, and Aly simply cannot be rescued easily.

Frantic, the kids come to believe that their only hope is to find the rest of the lost Loculi in order to rescue Aly. With that, it’s off to the Temple of Artemis at Ephesus, where they have to face down the armies of Artemis, and then to the Lighthouse of Alexandria, where they wind up in the belly of an unimaginable beast. And then finally, back to the Karai Institute for a final trip back to Atlantis to save Aly, the world, and themselves.