Today I will explain to you how to create a tunnel under a water surface! First of all you have utilize a bug caused by water surfaces; if you align two surfaces over one another the one above will not act as water. You can go under it without switching to the swimming animation and you also will not drown. Now we create a small scene to showcase one possible example to use this bug for your maps. In this setup with two water sources on top of each other and the surrounding landscape to hide the upper water surface, we can create a somewhat realistic setup to work with. You have to use the “Delete Terrain Tool” to create holes in the terrain for your pathway.

In this case we use some of the bunker parts to make a tunnel under the fake water. Keep in mind that the upper water source is just for aesthetics and it will not be possible to swim in there..

As found on Youtube