For streaming endless terrains, yes you can do that with Unity using AssetBundles, if you have a Pro license. See this sample.
As for explosions, that seems to be a separate question, please post it as such, and explain in more detail what you actually need.
answered Jun 23 '10 at 03:05 PM
jonas echterhoff ♦♦
Yes. You can create terrains and modify their heightmaps at runtime
You'll need to check with the Terrain and TerrainData classes to see how you can use them
answered Jun 22 '10 at 11:28 PM