Check around for glowing green rocks.

I may have missed something, but.....

How old are these impact craters? How does that compare with the age of the mounds in question? If they aren't pretty much contemporary, how would the local people identify a particular divot as being from an impact? Just on account of suspicious roundness? If so, wouldn't they need to be able to overlook the area from a height to see the roundness? Are there hills nearby to spot it from?

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It is hard to convey five-dimensional ideas in a language evolved to scream defiance at the monkeys in the next tree. -- The Science of Discworld III: Darwin's Watch, by Terry Pratchett, Ian Stewart & Jack Cohen