This is the Siberia tunnel exit, part of the Silver Rain's massive tunnel network. It's rather well hidden, which explains why the OA hasn't simply destroyed it. This is where Silver Rain mercenaries emerge to do whatever job they were assigned. When they are finished, they go right back through. Hidden just inside the tunnel is a Gordos that jams all sensors and comms of non-Silver Rain Zoids, allowing the tunnel to remain a secret.