Here's a picture of a large hole in the ground known as the Bingham Canyon mine:
It was dug up to mine a large amount of copper ore.
In South Africa, there is the Tau Tona mine, where gold ore was mined:
And here is the Mirny diamond mine in Siberia:
As you might have guessed, modern civilization needs a lot of minerals, and in order to preserve them, we have literally reshaped the earth's surface. To the extent that geologists believe that our current age must be considered the beginning of the Anthropocene era: a geological age in which human activity has left traces on the Earth that will be detectable for thousands or millions of years.
Well, if there had been a technological civilization on Earth in the last million years, where are the holes? Even if they were filled, they would be geologically different from the surrounding rock. In fact, our search for minerals has led to geological investigations that would reveal such anomalies if they had been found.