HOW does Denver get the "Mile High" name?? When I went...

I had NOOOO idea it was so flat just East of the city. I mean...SEE FOREEEEEVER flat! I had always envisioned Denver and the whole area at least in hills. It was funny when we stepped outside the Denver airport to the Rental Car bus station, we were facing East and all I saw was....FOREVER! Looked West and BAM!!! There was the mountains. It was VERY interesting to see the sharp contrast. When we drive to North Carolina or North Georgia, we see some hills before the bigger stuff.