@SomeoneElse I can find no such "golden rule" in the wiki, and I bet there never was. A rule that is in the Wiki is the "on the ground rule" (https://wiki.openstreetmap.org/wiki/Disputes#On_the_Ground_Rule). I'm the only mapper who has been inside this sewer and knows what it looks like. The others are armchair mappers who have not even been at the entrance. I already explained to them that this is a sewer and why it's not a tunnel, but they didn't even care to read, and were still talking about a brook. It's ridiculous.

Are you sure you got your forum link right? It does not lead to any discussion about sewers or tunnels. Both are not specific to Austria anyway. The lack of a forum or ML discussion on this topic indicates that the people who started quarreling aren't independent of each other, but it's only one (Negreheb) who brought in all of his friends. Otherwise they couldn't have posted all within just a few hours. There's no way they would ever have come across this sewer, let alone all at the same time.

Do you think this should be a game of "You bring all of your friends, and I bring all of my friends, and then let's fight" ? Can't we just choose the tags based on the facts?

I know what a tunnel is, and what is not. I have been surveying natural and artificial caves for 13 years, and I'm officially responsible for man-made underground facilities at the region's cave club. To make sure, I asked a club mate who has been involved in tunnel construction planning, and he confirmed that this is not a tunnel. I can forward you his mail. It would be easy for me to outnumber Negreheb and his friends by bringing some of the 500 members of our club, but again, that would be ridiculous. And frankly speaking, my club mates wouldn't love to waste their time on such a needless debate. Most of them are hard-working people who have better things to do.