George Lucas told SCI FI Wire that preliminary research has begun on a story for a possible fifth Indiana Jones movie, but added that such a film is one of dozens of potential projects currently crowding his schedule.

“It sits on the shelf there as one of 50 projects that I have to deal with,” Lucas said in an interview on Aug. 4 at his Big Rock Ranch in San Rafael, Calif., where he was promoting the upcoming computer-animated feature film Star Wars: The Clone Wars. “And if I can come up with a story …”

But, Lucas added: “It’s very hard to come up with stories for that thing. You know, It’s really impossible. Because it has to be real. I has to be something that actually happened. It has to be something people know about. It has to be supernatural. It’s a really difficult research project. Which they’re researching now. You know, and last time it took 14 years. So … ”

Lucas was referring to this summer’s fourth Indiana Jones film, subtitled The Kingdom of the Crystal Skull, which was in development for more than a decade before the pieces finally came together.

In recent press interviews, Lucas has said that the three principals–himself, director Steven Spielberg and star Harrison Ford–all have to agree on the idea for a fifth film to move forward. He has also said that Spielberg is more amenable to the idea of a further installment than he was last time around.