Brief but clear daylight sighting near Lake Tenkiller witnessed in Cherokee County, Oklahoma

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In the summer of 2000 I went with my stepfather and his friend to some land (40 acres) that my mother inherited from my grandfather. It's in a fairly remote and sparsely populated area near Lake Tenkiller. My grandfather had bought the land because we went to the lake multiple times every year when I was growing up, and fancied building on it. He passed away having never done anything with the land. We were going to clear a 'road" from the public road to as far as we could go to the back of the property. That meant clearing brush and cutting trees (with chainsaws). We arrived around 9:00 am and began work. We took a break for lunch and I decided to explore the rest of the property while they sat in the truck and ate. At the back of the property (approximately 150 yds from where we were working) the land dropped steeply into a canyon/ravine. It was pretty clear all the way to the bottom and probably 80-100 feet of vertical drop. As I neared the bottom I saw what looked like a cave on...