your using a blend texture only.....all terrain needs to have a nonblend texture under all blend textures.
what does this mean?

If you are using a blend texture you also need a normal texture under it because otherwise you will fall through the ground and if you blend with only one texture you will see through the ground unless like starkk said you have a normal texture under it, instead of looking through the ground you will then see the normal texture.

It sounds to me like you are using an Alpha Texture. Try using a texture that has an "@" symbol in its name...