According to recent documentation, there is nothing recorded in the tunnel but Fog, and certain tunnel walls that lead to dead ends or void dropoffs. However, it seems to be that it's a tunnelling system similar to The Maze, but undoubtedly small, and clearly unfinished.
It's speculated the River Tunnel will play a vital role with "boats" (a widely anticipated feature that may be added into the game) and will make the Ocean Cove accessible to users.
The tunnelling system also has its own music, which is the same music that plays in the vicinity of the Green Box.
Accessing The Tunnel Edit
The inner tunnel cannot be accessed without strategic use of glitches and/or exploits.
It's been recorded by several users that the tunnel was missing most of its areas upon disconnection or glitching. Thus, it is speculated for some reason the tunnel doesn't load completely upon Server Initialization.