[scythe]

Mines seem like a possibility. Hmm, this kind of rock is usually associated with these minerals, but not when these weird streaks appear.

But radionuclides are the ultimate signals. Palladium-107 is a byproduct of nuclear weapons use that becomes incorporated into palladium-bearing rocks and decays to silver, which can be detected by the presence of Ag-107 in palladium ores lacking other silver isotopes. Remaining Pd-107 can be used to estimate the date at which the palladium was deposited or the ore can be dated by its depth. So we can exclude the use of uranium-based nuclear weapons by previous inhabitants going back about... 60-80 million years since the half-life of Pd-107 is about 6 million years.

In theory we could have tried to detect whether nuclear weapons had been used on Earth in the last 120-150 million years by looking for iodine-129 in the oceans, but we already blew up too many bombs to confound that experiment. Finding iodine-129 some place we don't expect it would be interesting.

[caf]

Presumably we could look for ¹²⁹I in deep Antarctic ice cores?