Now I promise this one is going to be tougher - drop a link to the Google Maps location where you think this tunnel might be.
Please note it's not a sewer line/tunnel or anything like that - it's completely accessible by someone on foot and it will be wet/soggy/muddy at certain times of the year just by the nature of where it's located although I've never seen water actually flowing through it (just pooling). Cliff swallows also build their nests inside of this tunnel which is about 110 meters in length...