The US is less than 250 years old, but some of its most important archaeological sites are older than the Viking seafarers, the Roman Empire, and the pyramids.

Many help tell the story of how the first humans came to North America. It's still a mystery exactly how and when people arrived, though it's widely believed they crossed the Bering Strait at least 15,000 years ago.

"As we get further back in time, as we get populations that are smaller and smaller, finding these places and interpreting them becomes increasingly difficult," archaeologist Kenneth Feder told Business Insider. He's the author of " Ancient America: Fifty Archaeological Sites to See for Yourself ."

Some sites, like White Sands and Cooper's Ferry, have skeptics about the accuracy of their age. Still, they contribute to our understanding of some of the earliest Americans.

Others are more recent and highlight the different cultures that were spreading around the country, with complex buildings and illuminating pictographs.

Many of these places are open to the public , so you can see the US' ancient history for yourself.