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One day, a race of hyper-advanced aliens are on a stag weekend to Earth, and for a joke they decide to 'tag' every atom of every material that humans have 'worked' through chemical processes (eg building materials, artificial fibre clothes, electronics, etc, but not most crops or animals) in a way that leaves them permanently identifiable but doesn't change their chemistry, for instance by exciting the quarks in their nuclei to a highly meta-stable excited state.

Tens of millions of years later, another equally-advanced race attempts to search for evidence of this action, for which they need to collect as much of the contaminated material as possible. Where on Earth would they find the greatest concentration of tagged particles? Washed out to sea and deposited in ancient river basins? Buried under some meters of new material on the sites of old cities? I assume that after several whole geological ages there would be nothing left recognisable on the surface.

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  • $\begingroup$ wherever the humans are, or are humans extinct? $\endgroup$
    – John
    May 24, 2023 at 20:13

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We only need to look at modern materials, since the current economies dwarf anything from the past by orders of magnitude.

The biggest bulk material we use is water, followed by oil. We process more oil than anything else combined and we use more water than we use oil, for the simple fact that it is cheaper to come by. The only disqualifier for these atoms is that the remains are spread out though the atmosphere and oceans. So while being the most, there are not so many processes that concentrate these materials.

The Vale Northern System

Disclaimer, I did not do actual research, but the industry that moves the most materials in a concentrated way is mining. Iron and aluminum mining leaves large amounts of processed ore that are dumped in a single location. Search for aluminum processing waste and find that for every ton of aluminum 2-2.5 tons of waste 'mud' is produced. The iron production is about 10 times the aluminum production and about 5 times the plastic production.

Then garbage dumps and cities remnants. These are difficult. It is difficult to gauge how much material is collected in concentrated places for these categories. And also how much it will erode after millions of years. garbage can oxidize and concrete (calcium) can leach into the ground water and erode.

Then mega-structures. Large dams contain lots of material. And while always close to a river which might erode it over time, a part of the dam might not be in the direct path of the river and stay more or less in the same place. Even though it will flatten out a bit.

So my bets are on the biggest mines. Although city remnants might win if concentration is less important than total amount of material in a single region. As cities are more spread out than the piles mines leave behind. The front of the mine tailings pile at the Iron King Mine and Humboldt Smelter Superfund Site in Yavapai County.

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