In my area there was a lot of mining over the past centuries, but completely closed down since long. Scattered all over the area you find lots of relics from these activities, most of them easy to tag.
Recently I ran into the remains of an old ore cable-car line transporting the ore from various mines to the smelting plant. The line has gone since long, but alongside one can still find the concrete foundations of the former steel pylons (which have been removed completely). Most of these foundations are located in forest areas today, some on meadows or farmland. One set consists of 4 - 6 massive concrete blocks, some 1x1 m base and up to 2 m high, surely visible OTG for the next hundred years and therefore well worth to be tagged. Another type are the foundations of the former mine shafts which are even bigger.
There are some 30 man_made=foundation tags acc. to taginfo so far which makes me wonder if there might be another tag for those - there must be thousands of such objects worldwide imho.
Thanks for your input …