How to make tunnels in my map?

I’m a noob trying to make my first test map. I have a mountain on my map and I’d like to make a tunnel through it, but I have absolutely no idea how to go about that. Ideally I want the tunnel to end in a lake so part of it would be underwater. Is this even possible? None of the turorials I looked at have covered something like this. But maybe I’ve just been looking in the wrong places…
Thanks for taking time to help me in advance.

use the Visibility Tool under Landscape.