The stakes have never been higher. If Percy Jackson and Annabeth fail in their quest, there'll be hell on Earth. Literally. Wandering the deadly realm of Tartarus, every step leads them further into danger. And, if by some miracle they do make it to the Doors of Death, there's a legion of bloodthirsty monsters waiting for them. Meanwhile, Hazel and the crew of the Argo II have a choice: to stop a war or save their friends. Whichever road they take one thing is certain - in the Underworld, evil is inescapable. The fourth book in the bestselling Heroes of Olympus series - set in the explosive world of Percy Jackson. The Greek and Roman demigods must work together to seal the Doors of Death - and help Percy and Annabeth escape the Underworld in the process. Keynote The fourth book in the bestselling Heroes of Olympus series - set in the explosive world of Percy Jackson. Heroes of Olympus is Rick Riordan's Number One Bestselling series featuring Percy Jackson Puffin sales of the Percy Jackson series are now at OVER 1,000,000 The Mark of Athena was a No. 2 bestseller and has sold over 64,000 copies (TCM) in Hardback and counting!