|location = underground|
|Indicate a given feature is buried or dug under the surface of the ground|
|Used on these elements|
|Status: in use|
|Tools for this tag|
This tag is suitable for any feature located under the ground surface and not actually visible at the time of the survey.
Concerns with verifiability
As verifiability is an important concept in OSM, buried features may not comply with this principle.
Highway/railway/waterway tunnels won't cause any trouble since they got pretty al the time open ends and are clearly visible or accessible by car, train or by foot.
When it comes to pipes or cables installed by mankind, we can find some markers indicating the feature is actually there without seeing it. Such things should be serious enough to allow the underground feature to be added on OSM.
You should refer to Verifiability guidelines page at every survey you make of underground equipment.