The next step is to change the elevation of the bottom of the Aquifer. The aquifer has a thickness of 12 m so the elevation of the bottom of the aquifer will be calculated by subtracting 12 from the top of the model.
Color the grid (Data|Color Grid...) with the Aquifer_Bottom data set. Be sure to click the Apply button. Then select Data|Edit Data Sets... Select the Aquifer_Bottom data set (under "Required|Layer Definition") The Default Formula is -10. Click the Edit Formula button to open the Formula Editor. On the right, double click on Model_Top (under "Data Sets|Required|Layer Definition") and then type -12 so that the final formula becomes "Model_Top - 12." Click the OK button to close the Formula Editor and then click Apply in the Data Sets dialog box.
The grid may disappear from the front view of the model because the bottom of the grid is above the top of the front view. To get it back, select Navigation|Go To... Change to the Block tab and uncheck the Top and Side check boxes. Click OK and the grid should be visible in the front view again. Another way to do much the same thing is to select Navigation|Restore Default 2D View 
.
It is instructive to look at different cross sections of the model in the side view. First stop coloring the grid by selecting Data|Color Grid..., selecting "none," and clicking the Apply button. Then locate the cube with the blue square on its left face in the top right corner of the side view. The position of the square in the cube indicates the selected column. In the top view, the grid lines around the selected column are drawn with blue lines. Hold down the shift key and click on the cube to the right of the blue square. The selected row will increase by 10 and the blue square in the cube will shift accordingly. Observe how the side view changes appearance depending on which row is selected. It can be instructive to see what happens when the shift key is not held down when clicking on the cube or when holding down the Ctrl key when clicking on the cube. For more information see the description of the Selection Cube. Similar experiments can be performed with the front view of the model.
