Vortex Gaming

· In Cities: Skylines 2, tunnels may seem simple, but they are a source of frustration for many players.
· There are many questions and complaints, especially about the difficulty of creating clean, 0% slope entrances.
· This guide introduces a consistent method for creating smooth tunnel entrances.
· It's important to flatten an area wider than necessary in front of where the tunnel entrance will be, to secure working space.
· Later, you can naturally tidy it up using the soften terrain tool.

· The key is to create a slope so that the tunnel entrance blends naturally with the terrain.
· Use the slope terrain tool to set the target height. Typically, right-click 10-20m above the road to set it.
· Drag from the road to the set target point to create the slope. The further the target point, the gentler the slope.
· Extend the road straight along the slope you just created.
· You need to adjust the height so the slope is 0-1% (ideally 0%).
· Lower the Elevation Step to 0.18 or 0.1, then use the Page Up/Down keys for fine-tuning to create a perfect 0% slope.

Step	Description
1. Set Target Height	Right-click 10-20m above the road to set the target height
2. Create Slope	Drag with the slope terrain tool from the road to the target point (further distance means gentler slope)
3. Extend Road	Extend the road straight along the slope you just created
4. Adjust Height	Adjust height so the slope is 0-1% (ideally 0%)
5. Fine-tune	Lower Elevation Step to 0.18 or 0.1 and use Page Up/Down keys

· To determine where the tunnel will emerge, lower the network's elevation below the tunnel threshold of -2.5m.
· Use the simple curve tool mode to test the underground section and roughly determine the exit location.
· Check the elevation of intersecting networks (e.g., 9m).
· For the exit slope, set a target height on the hill (e.g., 20m above the intersecting highway).
· Similar to the entrance, flatten a wide area, set the target height 10-20m above the road, then prepare the exit slope with the slope terrain tool.
· It's crucial to prepare the terrain first to ensure the tunnel entrance remains flat.

· Tunnel exit construction follows the same process as entrance construction.
· Extend the road straight along the slope you just created, and adjust the height so the slope is 0-1%.
· Tunnels can be connected from either end.
· Perform tests to ensure the slope isn't too steep.
· Check alignment and neatly connect the road from the entrance to the exit.

· Once the tunnel entrance and exit are connected, you need to clean up the terrain in the work area.
· Use the soften terrain tool to make the surrounding terrain look natural.
· Near the tunnel entrance, reduce the brush size to avoid ruining the carefully crafted slope.
· This process allows you to complete a clean 0% slope tunnel entrance without depressions, exposed tunnel walls, or terrain tearing.

· When building tunnels, you should avoid several common problems.
· If you don't disable Anarchy mode or force its use, the likelihood of problems increases.
· If you forget to turn off retaining walls when building the underground tunnel section, the tunnel walls may protrude above ground.
· Upgrading tunnels can also cause problems. It's best not to modify a tunnel once it's built.
· If changes are necessary, it's often better to completely demolish the underground section and start over.
· While all these steps might seem difficult, the most important thing is to create a good entrance slope. This is the core of tunnel construction.

Action to Avoid	Potential Problem
Forgetting to disable Anarchy mode	Increased likelihood of unexpected problems
Forgetting to disable retaining walls when building underground tunnels	Tunnel walls protruding above ground
Upgrading tunnels	Increased likelihood of problems (rebuilding recommended if changes are needed after initial construction)