@SomeoneElse: tunnel=* is defined in the wiki as an "underground passageway". As a native English speaker, would you say that a cul de sac is a passageway? Is it a passage if all you can do is return to the starting point?

If the anwer is yes to both questions, I can try to write a clarification proposal (similar to what I've done for natural=cliff), but it will take me at least a week because I will be handicapped from chemotherapy.

@KaiRo: Why would it be visible on the surface if layer<0 or location=underground is set? Are you talking about Carto rendering? Maybe it's just a Carto bug? Or location=underground (which is currently not set) fixes Carto rendering? If it's not about rendering, but just about tagging, then I'm curious if you want to make tunnel=* compulsory on all linear underground objects, or if there are any exceptions. The next question would be whether you wish to deprecate location=underground in favor of tunnel=*.