Planetary Scars

… was once home to a race of reasonably intelligent beings who destroyed themselves when they were seemingly only decades away from space flight. The planet has recovered from what must have been immense acts of senseless destruction, but it still bears very visible scars.

One of the few remaining ruins on the surface holds a reasonably well-preserved stone monolith inscribed with a message, quickly decoded by …. It is a warning, from the last of the extinct species to whoever they believed would arise after their passing. It was the development of an "ultimate weapon" that was their downfall; some quirk in the psychology of this species evidently rendered deterrence through mutually assured destruction impossible.

Survivors, before dying of unknown causes soon after, hid this weaponry deep underground as destroying them was difficult, and left this message dissuade us from going looking for them.