Terrain Texture Glitch?

Haven’t found a solution anywhere unfortunately, but my terrain looks like this:

This is without light. With light, it’s the same except the terrain is white and that purple inside is blue.

It acts a bit like a window. When I move the terrain up, it’s like there’s a void on the other side and I just discover more of it, the previous top becomes the bottom, if that makes any sense.

What version of unity are you using? This happened in the last few patches of unity 5 versions. It has something to do with the shader. Try going to terrain, inspector, click on the cog, and change the shader to diffuse. Does that have any effect?