Grit Engine is very green, Use OGRE 3D for Graphics, Can use in Windows or Linux, Don´t have Terrain Editor you need to create your own terrain programs how GeoControl,World Machine with 3DS Max,Maya,Zbrush or Sculptris. if i remember good the main core is written in Lua and C++ for graphics for OGRE 3D, with Bullet Engine for Physics Only have importer Blender and 3DS Max, The importer of Blender is good but 3DS Max is rustic. But have a good Stream System, include a Day/Night Cycle, Good System of Light,Shadows.The Documentation is little,Forum have low people but have good people help you in what they can You can use the Engine but need good knows about programming in C++ and Lua, or you can learn how work a engine and create a good stream for your own engine.
As far as I know none of the engines above have full support for physics included. There is this discussion on how to do fluids in bullet though: http://bulletphysics...opic.php?t=4210 Its old but might still be relevant