I would guess that things like massive landfill dumps would be almost impossible to wipe all trace of. There’s no natural process that can collect that huge number of different chemical elements in such high amounts into a single location. So even if literally everything manmade has broken down to its consistent elements, the presence of a mile-wide radius plot of earth containing every solid element in the chart would be a clear indicator that an advanced civilization must have been there.
I would guess that things like massive landfill dumps would be almost impossible to wipe all trace of. There’s no natural process that can collect that huge number of different chemical elements in such high amounts into a single location. So even if literally everything manmade has broken down to its consistent elements, the presence of a mile-wide radius plot of earth containing every solid element in the chart would be a clear indicator that an advanced civilization must have been there.