WASHINGTON — Oscar-winning director Kevin Costner has turned novelist in a big way, kicking off with an adventure yarn aimed for the ages that weighs in at a door-stopping 784 pages.

“The Explorers Guild,” which the 60-year-old “Bull Durham” star co-wrote with Jim Baird, is set against the backdrop of World War I. It tells of a group of adventurers on a global quest to find a mythical city.

“[The] idea that this book is going to be read 150 years from now … might be lofty,” Costner told reporters in Washington on Friday, flanked by Baird and illustrator Rick Ross.

“It might not take its place among the books that we love so well, but we wanted it to.”

“The Explorers Guild,” Costner’s first novel, incorporates comic-book style illustrations by Ross, an artist Costner said he found on the classified ad website Craigslist.

“I love the idea of the physicality of heroes trying to use their wits and their resources to get themselves out of trouble,” said Costner, who compared the book to Jules Verne’s “Twenty Thousand Leagues Under the Sea.”

Costner pitched the story as an animated film about eight years ago but said Hollywood was not interested. He ended up using his own money for the book project.

“I’ve found that some of the things that I have loved the most I have had to pay for,” said Costner, who also funded his film “Dances with Wolves.”

The movie won seven Academy Awards in 1991, including an Oscar for Costner in his debut as a director.

Subtitled “Volume One: A Passage to Shambhala,” Costner and Baird said “The Explorers Guild” might be the first of a series.