The Tunnel is located in the northwest corner of the Swamp area. It is restricted on all sides with barbed wire and irradiated water. A breach in the fence makes the area accessible. To reach it, search for a corpse marked on the PDA and walk left at the barbed wire fence into the reeds.
The corpse is part of the side mission Clear Sky scout's PDA, see List of optional missions in Clear Sky.
The cave next to the bridge looks very similar to the Dangerous Cave.
The location appeared also in the Swamp (build 1935) behind the intact bridge, but access was also blocked.