Dark Day in the Deep Sea (Magic Treehouse Merlin Missions #11) by Mary Pope Osborne

The #1 bestselling chapter book series of all time celebrates 25 years with new covers and a new, easy-to-use numbering system!

The magic tree house leads Jack and Annie to a tiny deserted island in the middle of the ocean. The brother and sister team are soon rescued by a ship of explorers and scientists who are looking for…a sea monster!

Formerly numbered as Magic Tree House #39, the title of this book is now Magic Tree House Merlin Mission #11: Dark Day in the Deep Sea.