Tunnel1

The first section of the Bergmann Tunnels that Mark is assigned to inspect. This area is quite dangerous and in a much greater state of disrepair than the dam was. The tunnels are the first place in the game that contains an action sequence, in which Mark must perform a task in a limited amount of time.

Story
Mark enters the tunnels, and begins his sordid investigation-escape of the system. The area is abandoned, under extensive disrepair, flooded in several areas, inoperative in most places, and carelessly wired. It's here that Mark can find documentation about Erik and Robin, former employees of the Bergmann Group assigned to the tunnels, as well as another audio log by Alex Hartman, seeking an alliance with Eric Kleinmann.

After navigating flooded corridors, mazes of pipes, and an unfortunate shortage of valve handles, Mark attempts to open the gates at the end of the main tunnel. One of the gates break, causing a chain reaction that violently disturbs the already barely standing concrete structure. The ceiling begins to cave in, and Mark will barely have enough time to open a flood door, take an elevator and escape the tunnels.

Bergmann Tunnel
The main tunnel was being maintained in several places by Erik and Robin, with Robin doing very little to contribute. Mark can take two different routes to the elevator, although he can explore the entirety of the tunnels if the player wishes to maximize their photography score.

Door B2
A normally locked door can be unlocked by finding a hidden fuse in the break room storage area, and key underneath a can on the east side of the tunnel. Entering the code (4027) and proceeding loads a secret map. This mysterious tunnel also serves as an alternate means to escape the collapsing tunnel. Be warned though, that taking this path will skip the Pine Fell Water Treatment Plant.

Trivia

 * The code left on the flood door alarm control panel is 0451, a reference to a popular code used to open the first door in games made by Looking Glass Studios and Ion Storm.