Old Technical Tunnels
From Drug Dealer Simulator Wiki
The Old Technical Tunnels are a route between Sector A and Sector B. They can be accessed while wearing a backpack, but there's a cost. Every day you want to use the tunnels, you must pay a construction worker an amount of money to open them for you.
The tunnels are located slightly south east of Two Towers, and north of Government Docks.
For a free, but more limited alternative, see Sewage Hatch.