Dark Day in the Deep Sea (Magic Tree House, No. 39)

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 the crew isn’t looking for the secrets of happiness. The crew is looking for . . . a sea monster!