Cave beneath the sea

Summary: "Seeking to recover the shards of King Arthur's sword Excalibur before Rex Major (the ancient sorcerer Merlin in a modern-day guise), Ariane and Wally have traveled around the world, and their quest is not yet over. Ariane's magical powers as heir to the Lady of the Lake, combined with Wally's clever thinking, have enabled them to hang on to two of the three shards they've located so far; but Rex Major still has one shard of his own, and will stop at nothing to claim all the pieces for himself, and use its power to rule Earth and invade the magical realm of Faerie. And now Major has his eyes on a big prize: Ariane's mother, who went missing after she refused the power of the Lady of the Lake. Thanks to Wally, Ariane knows her mother is alive -- and that Rex Major is trying to find her. If he succeeds, Ariane fears she'll have no choice but to surrender the two shards she and Wally possess, because she'll do anything -- and give up everything -- to have her mother back with her again. In Cave Beneath the Sea, Ariane and Wally race to the Caribbean as they try to find Ariane's mother and the fourth shard of Excalibur before Major. As they struggle to stop Major, Ariane and Wally face desperate danger ... and must make the most difficult decisions of their lives."--Publisher's description.