Filming John Carter of Mars

Tusked men with six limbs, like insects eight feet tall; flying galleons and airships with dragonfly wings; cities marching across arid deserts on mechanical treads; a hero who can leap 30 yards at a stride and fell monsters with a blow. A Princess Of Mars has been, for 100 years, not only the most surreally thrilling adventure story but also the archetypal Unfilmable Book.

Even when the author's Tarzan tales were cinema's hottest property, and Ray Harryhausen's marvellous animated monsters were wreaking stop-motion carnage in the Greek myths, every Hollywood producer accepted it was impossible to adapt Edgar Rice Burroughs' Martian series.

There was an extravagant pleasure in reading the books and knowing that no movie director, with a budget of however many hundreds of millions, could match one human imagination.

And now Disney has done it, with computer animation so astonishing it's scarcely believable that the apes as tall as trees and the razor-fanged slug-dogs are not real. What's best about the film is that the director and producers are evidently real fans of the books. The CGI marvels are not only precisely rendered in every hair and scale, they are also perfectly faithful to the text.

The landscapes of Mars, its architecture, its monsters and villains, its synthesis of imperial Rome and Arthurian chivalry, and the brute nobility of its hero are all familiar. This is Conan the Barbarian teleported into Star Wars.

So it's a jolt to remember that John Carter arrived decades before either of them. This was the original sci-fi/ fantasy adventure, a new genre hatched fully formed, like a Thark from an egg. It's taken cinema a century to catch up, but I'm glad it has. Now I want to see another patently Unfilmable Book on screen – David Gemmell's siege epic Legend. Gods, men, angels and an army of a million demons... it's what CGI was invented for.