While Fox had hoped they'd get their own Harry Potter size family fantasy franchise with Percy Jackson & The Olympians: Teh Lightning Thief, with the film not even breaking $100 million at the US box office, it wasn't the success they'd hoped. It had been assumed that the middling grosses had killed off plans for a follow-up, but apparently not.
The LA Times reports that Fox 2000 is moving forward on a sequel to last year's Percy Jackson, and have hired screenwriters Scott Alexander and Larry Karaszewski to pen the follow-up. The: Lightning Thief was based on the first novel in Rick Riordan's five-book series, while the sequel will be based on the second, The Sea Monsters. The script is still in the early development stages, with no director attached yet.
Chris Columbus, who directed the first Percy Jackson movie, is attached as a producer of the sequel, but he won't direct. Original cast member Logan Lerman will reportedly return for the sequel (presumably he's optioned for quite a lot of them in his original contract). The story revolves around the title character and his friends, who head under the sea to find the mysterious Golden Fleece.
It may seem off to have waited over a year to get this one going, especially with the cast quickly aging, but it seems Fox has decided that despite the modest grosses of the first film, with the books still immensely popular and with The Lightning Thief's home entertainment and TV cash taken into account, it's worth a punt on the Sea Monsters.