The River Tunnel is a geographic feature located near the Bridge. It is, as its name suggests, a tunneling system boring under the swamp.
There is nothing recorded in the tunnel except tunnel walls that lead to dead ends or void dropoffs. However, it seems to be that it is a connector to a tunneling system similar to The Maze, but undoubtedly small, and clearly unfinished. It's speculated the River Tunnel will play a vital role in naval transport (a widely anticipated feature) and will make the Ocean Cove accessible to users. The tunneling system also has its own music, which is the same music that plays in the vicinity of the Green Box (Kevin Macleod - Swimmey Texture).
During the time of Alpha testing, the tunnel could be swam into because the ocean did not damage the player. The tunnel may have looked different during this time, but no Alpha images of it exist.