We went into the tunnel twice. In the first attempt, we could not spot the ventilation shaft - we were expecting a pillar of sunlight inside the tunnel, but there was only absolute black darkness. Honestly, we got a little nervous and left the tunnel.
Once outside, we chose to climb to the tunnel top to have a look around. This is the view of the embankment east of the tunnel - there is still quite some climbing to be done before reaching the top of the hill.