New story idea

So, I had an idea for a story, which kind of draws on other ideas a bit, and is kind of new. The other ideas never really got all that far, and though I hope this one will, there's no guarantee. At the moment, it seems to be a bit hit and miss whether I actually continue a story or not. I will finish The Desert Rat at some point though, because I like the characters in it, and I like the idea, it's just that I keep getting bogged down with other ideas.
Anyway...
I had this idea of a civilisation that lives below ours, a little like the fairies in Artemis Fowl. However, this civilisation is at threat because of rising sea levels beginning to flood some areas of Inside. Three years ago, a were-centaur by the name of Tatharn was sent out. His goal? Study Outside and see if it is habitable once again. At that time, Inside was not under threat by flooding. Since he left, the flooding started, but not only that, an extremist group of 'politicians' called the Elva took over control of Inside. They want him to report that Outside is habitable, which, as we all know (seeing as we live Outside) it is, and then they want to launch a war, in collaberation with the USSR. It'll be set in the 80s (I think, because I need a communist nation, although maybe I could use China... worth a thought I guess, it might be easier for me to set it now), who they've already told about their existence. The Elva is basically based on Communists, and emobdies that kind of totalitarian society.
However, the Elva don't know exactly when or where Tatharn is due to return--only two people know that. These two were once his commanders, although they have now been stripped of their rank and are ordinary workers. They meet Tatharn, and tell him about what's happened. He reports that since they went Inside, several thousand years ago, the earth has once more become inhabitable, and that a race of creatures has taken it over. Tatharn persuades his two friends to leave with him, and to go find help from Outside, to deal not only with the flooding, possibly by integrating those who live Inside (elves, centaurs, possibly others too, I haven't yet decided) with those who live Outside. Inside has a lot to offer Outside--they have developed technology for harnessing the power of the earth, which they call Mayjiche, or, the human equivilant: magic. Their weapon technology is surprisingly primitive in comparrison though. They still use swords and bow and arrows, although as both elves and centaurs can use Mayjiche personally, they are able to use these arrows with astonishing acuracy and power over huge distances. If it came to a war, who knows who'd win? But Tatharn and co are hoping to persuade the US, or perhaps Europe, to lend a hand and defeat the Elva, preferably without even having to embark on the path to war, though this seems unlikely. The Elva, on the other hand, are determined to embark on war, along with China, to subdue the rest of the world. After all, they have the might of previously unseen species, and previously unheard of Mayjiche on their side.
So, that's my idea. Before I get really stuck into it though, I'd like to finish To Steal Faith, which is nearly, nearly finished now...