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 breech 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.
This place is of interest, as there is a cave left of the bridge, where you can find a Veles detector early in the game and a Gravi in the bucket. The Detector is hidden behind the mattress.