[typewithrhythm]

It seems like that's an impossibility, since you would need to find something in the current era that has been abandoned, rather than decommissioned...

There are a few examples that might fit, some earthworks, (tunnels, breakwaters, dams) and navigation markers come to mind (costal, but we also put retro reflectors on the moon).

[BlueTemplar]

Yeah, I was thinking about Vauban's fortifications, but if any of those had been abandoned, it's specifically because they would have been mostly useless after WW1 (= non-functional).

Hmm, any Vauban-like fortifications in Ukraine that would have suddenly found a new use since 2014 ?