The lake was dead calm, like a sheet of glass, when our friend called us to look at the water as there appeared to be what initially looked like a telephone pole/log in the distance blocking our path. We put the engine in neutral, and watched from about 300 feet as it (the log) appeared to be gliding straight along with no current on the water. I would estimate its size to be around 60-70 feet long. We watched for a couple minutes and all of a sudden, it was like it rolled towards the bottom of the lake from front to back.