In Too Deep by Jude Watson

In the 6th 39 Clues book Dan and Amy Cahill travel to Australia and Indonesia, following the trail of their parents, to keep finding clues. Along the way they meet a friend of their father's who hasn't been the best about keeping in touch and follow Amelia Earhart and see Krakatoa. An intriguing entry in this series, this book was entertaining and tested familial bonds in a variety of ways (between Dan and Amy and within the Lucian branch). Also--I'm getting more and more curious about Nellie Gomez.