Cannot invert or lower terrain

Hi all. I'm new to Unity and therefore just learning how to use Unity, but I have come across a problem.

I cannot lower or 'invert' the terrain past its surface. I have increased the terrains 'height' from 0 to 1000, which should allow me to 'dig below the surface' right?

Also, how do I change the terrains material? I want to change it from the default gray to a grass texture. I have tried going into terrain settings and clicking material but there is no grass asset.

Thanks in advance, Vixone.

 

Hi. To lower the terrain after you have raised the height, you hold SHIFT, and click and drag the cursor over the surface. To put textures on the terrain:
1. Click on the terrain.

2. Look to the right in the Inspector panel and click on the little button that looks like a paint brush.

3. Click the button that says "Edit Textures..." then "Add Texture..."

4. A little window should pop up in the top left of the screen. Click on the square where it says "Select"

5. Select your texture and then click "Add'

6. You can repeat these steps and add many different textures(dirt, rocks, sand) and paint your terrain using the different brushes in the Inspector panel.

Also the Material for the terrain is usually just a "physics" material. Basically changing the property of the ground to behave slippery like ice or another type of surface. I think the only way to use Materials as in combined textures/colors/normal maps etc. is by using Allegortihmic Substances but I haven't been able to get it to work on the terrain in the free version of Unity
Hope this helps you