The usual response to an ancient Earth civilization is “where are the ruins?” Fair question, but I think it starts in the wrong place. Earth is not a clean storage shelf. It grinds, buries, melts, floods, oxidizes, erodes, and recycles its own surface. Oceanic crust is especially bad for long-term preservation. NOAA notes that very little seafloor is older than 150 million years because older crust gets subducted and replaced. Schmidt and Frank’s Silurian Hypothesis paper made a similar point...