We like to think that we are an exception: An intelligent life that has developed a vastly complex industrialized civilization unlike anything seen before. But would we actually know if millions of years ago some other life forms had created huge cities, altered the landscape, and formed their own civilization? What would be left of their exploits for us to find?

This is the thought experiment that the director of NASA’s Goddard Institute for Space Studies, Gavin Schmidt, and professor of physics and astronomy at the University of Rochester, Adam Frank, have been exploring. They want to understand if any evidence of a hugely advanced civilization could persist tens of millions of years into the future and, if so, what form it might take. Their results are published in the International Journal of Astrobiology.

“Gavin and I have not seen any evidence of another industrial civilization,” Frank lays out from the onset. Since they have not found any pre-human civilization, they have been looking at what impact we as a species are having on the planet, and how long those impacts are likely to be detectable into the future. “These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall.”

While we have plenty of evidence of past civilizations in the form of statutes and foundations that date back maybe a few thousand years at most, things get far more opaque when the clock is rolled back by a few million or hundreds of millions of years.