The Underground is a Travel-Tube system. You have the ability to Open them up, then walk towards the tube itself. and hit the "Travel" option. Now you have the choise to move from your current position, to any other tube-travel system point on the map. Keep in mind, the costs can vary and are between 20$ ~ 300$ .
South east harbor.
North west harbor.
Military base island.
Here are two examples so you're aware of how they look.
|Tube System Closed||Tube System Open|