Scholastic, 2012.
The 39 Clues is a series of adventure novels written by a collaboration of authors, including Rick Riordan, Gordon Korman, Peter Lerangis, Jude Watson, Patrick Carman, Linda Sue Park, Margaret Peterson Haddix, Roland Smith, David Baldacci, Jeff Hirsch, Natalie Standiford, C. Alexander London, Sarwat Chadda and Jenny Goebel. It consists of five series, The Clue Hunt, Cahills vs. Vespers, Unstoppable, Doublecross, and Superspecial. They chronicle the adventures of two siblings, Amy and Dan Cahill, who discover that their family, the Cahills, has been the most influential family in history.
In Too Deep is the sixth book in the series. It was written by Jude Watson (who also wrote Beyond the Grave) and published on November 3, 2009.
Dan and Amy head to Sydney, Australia, to learn about what their parents knew about the 39 Clues from their dad's cousin, Shepard Trent. They discover that Ian and Natalie's mother, Isabel Kabra, has joined the hunt. Isabel tries to feed Amy to sharks, but Hamilton Holt foils her plot. The siblings survive Isabel's second assassination attempt and follow a clue to Krakatau (the ring of fire), where they meet Alistair. Isabel sets fire to their hut. Irina Spasky dies while saving Amy, Dan and Alistair. After the fire, Alistair shows Dan and Amy a message by Robert Cahill Henderson, revealing the clue, water. Near the end of the book, they suspect that Nellie is spying on them for someone else.