Looking out through a window in Carnac archaeological museum you'll see a small chapel on top of a big mound, which is actually a Stone Age grave, built about 6000 years ago. The mound is 125 metres long, 50 metres wide and 10 metres high.

It was excavated in the 19th century, and then they dug a hole from the top of it and came straight into the chamber. Nowadays you can enter it from the door that you can see on the bottom left photo. Unfortunatly it was not open the day I was there in May 2015. I have seen it from the inside about 20 year earlier, but then I didn't bring a flash for my camera, so I have no photos of it, which is a pity.