First, of course, you begin with a river and your roadway on either side of it. The roadway (or railway) may be a monorail, an avenue, or a plain old road, but each end must be placed directly across from each other.
Next, you lower the terrain on each side of the river to form the entry point of the tunnel. It is wise to make the lowest point of the hole as close as possible to the water level. It is also a good idea to make the entry point the same depth on both sides of the river. And of course, each entry point must line directly up with each other.
This is the most important part of the entire operation. You must carefully level the land across the river, so as to trick the game into making a tunnel there. This is done by selecting the mayor’s leveling tool; pressing shift+1 to make it as small as possible. Now you hold your mouse over the vertex indicated in red, and in one motion, click and move your mouse east. After this all you have to do is continue leveling the land across the river until come to your exit point, but make sure you don’t go past it!
The easiest part of the who thing is making the tunnel across your newly built dike. All you have to do is drag the roadway across the river, and boom, you’ve got your tunnel. But make sure there is that green highlighted section stretching from the entrance point to the exit point. If it isn’t, press esc, and raise the height of the wall between each entry point.
If you want this to look right, you have to do it right too. To make the underwater tunnel… underwater…, you have to level out the dike and let the river flow though again. The recommended way to do this is do select the mayor’s leveling tool, and drag it parallel to the river’s flow, starting in the water and going towards the dam, as shown in the picture.
The final step is… sit back and admire your creation! (Yes, I know it’s amazing, but you should limit your admiration time to 5 minutes max).