I can see a couple ways to go with this. "Journey to the Center of the Earth" meets "The Matrix." What if, by traversing the tunnels and while going deeper one discovers they're getting closer to the surface, eventually emerging onto a similar planet, but turned inside out: they've actually been living inside the walls of a Dyson Sphere and now they've emerged into the interior and find a super-huge landscape that surrounds a dwarf star? (Hat tip to Star Trek TNG episode "Relics.") Or, perhaps "The Lost World" meets "Stargate" and maybe a little "Contact." Spelunkers discover the tunnels lead to what may be the world's deepest heretofore unknown cave. Radar results are unbelievable--it's thousands of miles deep. A helicopter-like craft descends and discovers that as they pierce the earth's crust and begin to penetrate the mantle that the predicted heat is not present--odd. Further explorations show all known assumptions of planetary structure are missing. Why? Researchers decide to use automation to follow the cave to the bottom. A probe is dropped and suddenly disappears without a trace but with no sign of malfunction. Finally, a lone high-tech spelunker goes for broke (think James Cameron exploring the Mariana Trench) and deep-dives the cave only to discover that as one nears the center of the earth, one actually traverses space-time (yeah, a little John Carter here, too) and a new method of interplanetary travel is discovered. Planetary cores are connected—who needs spaceships? But, if we can go out, who could come our way? And why haven't they (or have they)?

Will, both are interesting sci-fi / action adventure spins, going really big with the story. And why the hell not? Hollywood certainly seems to be going that way!

Dumb Story Idea #22 The Chronicles of Narnia meets Arthur--Three children in WWII find a passageway to a magical world where a lovable drunk lives a wonderful life. Feel free to check out all the @DumbStoryIdeas on Twitter.

Oh, I thought you were talking about Arthur and The Minimoys for a second (a movie where a boy discovers there's a tiny world in his own backyard), though I suppose that would already be similar to Narnia.