I ran into rather unusual but common problem with Unity's Terrain. The default terrain starts at height zero (0). This poses a problem when you want to create trenches and creates a greater problem when you have already sculpted the height of the terrain and now need to sculpt the depth. The solution to this problem is rather simple. Export the terrain as 8bit RAW file. Import the file into Photoshop. Create a new layer. Add a shade of gray that would represent the height increase. Change the blend type of this layer to Linear Dodge and then merge layers. Save the file and then import the new height map. This would remedy the problem.
The idea behind this solution is to add to all of the existing values of the terrain to create and offset from height zero (0). This maintains all of the existing sculpted terrain and allows sculpting downward or into the terrain.