From a friend:
It occurred to me that I’ve never seen a bird or animal (not ever!) in the tunnel. Also, before the tunnel was wired for radio reception (quite recently) you used to get a little blast of radio reception when you passed under what I guess must have been the vents. I also remember that the first trip I made through the tunnel was in 1965 with my dad during a trip to Liverpool to see what all the fuss was about. He said we had to hold our breath because we were going underwater – so I tried and, of course, failed.