I was an atheist while pursuing my degrees in Biology.
Because I was studying field Biology and not applied Biology, I had lots and lots of courses with 'field' components - meaning a part of the course included trips into nature to study. I remember one trip west of Altus to study Hispid cotton rats (you know them as 'pack' rats). We literally camped in a cow pasture with cows milling around just outside the perimeter in which we had set up our tents.
One morning I rose before everyone else and left my tiny one man tent (I still have it! I once weathered a tropical storm in it!) only to discover THE MOST amazing sunrise I had ever witnessed. There were more colors present than a rainbow it seemed and I stood there mesmerized by its glory. To this day I still remember the conversation I had in my head that began with, "This can't be an accident..." Unfortunately, it still took a decade or so beyond that before I became a believer but the first line of Psalm 19:1 describes my reaction fully.
The heavens declare the glory of God; and the firmament sheweth his handywork.As I was striving to hear God's voice over the years, this was an experience that never left my heart. God's handiwork IS a reflection of his glory and even the unsaved know this - He uses nature to help call us - we only need to open our heart and hear with more than our ears.