It's on my neighbor's property. She has 50+ acres that adjoin our 13 acres. In the 9 years we've lived here, I've occasionally run across a kid or two playing in the creek and I know area kids ride 4-wheelers back there... but, no, it isn't well known. In fact, I've asked my neighbors about it and none of them knew it existed. You have to be looking for it because the entrance was covered by honeysuckle and pricker bushes. We stumbled upon it by chance.
It use to be very cool looking when we first found it, but now the hill is sliding into the creek and the entrance (green mossy rocks) are gone. What the pictures don't depict is that it's actually quite high up and difficult to get to.
So I was just looking through my pictures from today to see if I had one that shows how it sits on the hill and how it's eroding away. I also pulled up my pictures from when we first found it on April 10, 2010. In my original post above, the two on the left (showing mossy green entrance) are from 2010 and the interior shot is from today. Now that I have the photos side by side I'm no longer certain it's the same stone chamber. The 2010 photos don't look like they have a stone floor and the rocks don't appear to be as brick-like. Maybe my daughters and I found a new one today?! If so, they must be very close together. My husband said he'll go down to the creek with us tomorrow and tell me if it's the same structure or not. He grew up playing in these woods (all of the collective properties together were his grandpa's before he parceled them out) and he said he can take me right back to the stones we found in 2010.
This is a photo of the entrance to the structure we saw today. It's taken from the creek looking up.

This is a photo of the exit to the structure we saw today. It's taken from the hillside looking down.
And if you're interested, I stuck my phone in and took a quick video so I could get a better look at it. Here's the video: