Magic Tree House Merlin Missions #11: Dark Day in the Deep Sea

Jack and Annie continue their quest for the secrets of happiness--secrets they need if they're going to save Merlin. But when the Magic Tree House leads them to a tiny deserted island in the middle of the ocean, they're not sure who needs help more--Merlin or themselves! The brother and sister team are soon rescued by a ship of explorers and scientists. But that isn't the end of their troubles, because the crew isn't looking for the secrets of happiness. The crew is looking for...a sea monster!