At the heart of any video game is its engine, and there is quite a selection right now: Unreal, Source, Frostbyte, and CryEngine to name a few. There is another you can add to list and it has a rather unique capability.
Outerra is the engine being developed for the game Anteworld, which will be a civilization building game when it is completed. The idea is that we had long since left Earth has returned to find all traces of civilization gone. Now humanity has to be rebuilt. Not exactly the most original story idea, but it does have a twist to it you may not have seen before; the entire planet Earth. The player can be thousands of kilometers above New York City or underwater off the cost of Japan and anywhere in between. (Instead of an open world it will be a closed manifold, for those of you who know topology terminology.)
You can download the tech demo of this engine right now, and its requirements arenâ€™t as bad as you may think (if you can play Skyrim, you can run the demo). As you explore the planet, the images and terrain will be downloaded when needed, totaling about 12 GB. The data are actual data for the planet, and where something is missing, the engine can fill it in using fractal patterns. For example, the original dataset for elevation has a resolution of only 90 meters, so the engine fills in the centimeters between with the fractal pattern.
The tech demo is free to download, but if you make a donation now (just $15) you get a spot in the beta testing and a copy of the final game when it is complete. The final game will cost $30, according to the site.