Countdown is the third book in the Unstoppable series by Natalie Standiford, confirmed on the Message Board by Admin Beth N. It will come with cards 479-484. The prologue and the first three chapters are revealed at 
Amy Cahill drinks the full strength Cahill serum to save her little brother Dan. Now Amy only has a week to find the antidote, or die. Pierce kidnaps Sammy and Nellie, and trap them both. Amy and Dan find the riven crystal, but Pony dies.
Admin Beth N revealed: "In Countdown, new author Natalie Standiford takes Amy and Dan to new extremes in their attempt to stop Pierce. With Dan's life on the line, Amy does the unthinkable—something that sends shockwaves through the Cahill world. Will she live long enough to regret it? Countdown is available May 2014, but don’t worry the Book 2: Breakaway release is right around the corner and will be in stores this February. I can hardly wait until then, but luckily I have all of your fan fiction to keep me reading ;)"
- On the cover, there are 8 letters visible around the disk which spell out ETHIOPIA.
- On the inside cover, there are 8 coordinates. To solve the puzzle, take each coordinate and turn it into the first letter of the city it points to. It reads STARLING.
- The page number code translates to "Hope Olivia"
- On Card 480: Axum Stelae, the code says: He will be the last president.
- Countdown is the 3rd one-word book title in The 39 Clues, after Shatterproof and Breakaway.
- It has a similar cover to The Black Circle.
- Referring to the secret code of Card 480: Axum Stelae, Pierce may succeed in making the United States a dictatorship.
|v • d • e The 39 Clues Books|
|The Clue Hunt||The Maze of Bones • One False Note • The Sword Thief • Beyond the Grave • The Black Circle • In Too Deep • The Viper's Nest • The Emperor's Code • Storm Warning • Into the Gauntlet • Vespers Rising|
|Cahills vs. Vespers||The Medusa Plot • A King's Ransom • The Dead Of Night • Shatterproof • Trust No One • Day Of Doom|
|Unstoppable||Nowhere to Run • Breakaway • Countdown • Flashpoint|
|Doublecross||Mission Titanic • Mission Hindenburg • Mission Hurricane • Mission Atomic|
|The Cahill Files||Operation Trinity • The Submarine Job • The Redcoat Chase • The Houdini Escape • Silent Night • Spymasters|
|Rapid Fire||Legacy • Ignition • Hunted • Crushed • Turbulence • Invasion • Fireworks|
|Additional Books||The Black Book of Buried Secrets • Agent Handbook • Midnight Ride • Worlds Collide|