In the Main Quest called “X13” in Dying Light 2, you will get the objective to find the tunnel entrance location. It will give you a rough search area but there won’t be an exact waypoint marker for it.
The tunnel entrance is located around the middle of the search area, which looks like a crater with rubble around it. Here’s the location (see white character arrow):
When you enter this crater, you will get a short cutscene, then you must defeat the enemies and go through the door to the tunnel. This will trigger the next objective.
For more Dying Light 2 guides, check out the Dying Light 2 – 100% Completion Wiki & Strategy Guide.
Thank you for the info about how to locate the tunnel entrance . God bless