Browsing Google Earth, I came across this area in Siberia.
It has a clear circular shape, showing concentric rings, and it's about 10 km across.
It's miles away from the nearest road, in a barren very rocky part of the world, so it's not human activity.
I've narrowed it down in my own mind, to either an ancient volcano, or impact crater.
Impact craters that show are very rare, so the odds are against it, but what do you think?
It's worth a look :
https://www.google.co.uk/maps/@70.9198194,103.0433649,15024m/data=!3m1!1e3?hl=en
Or on Google Earth :
70 deg 55 min 30 sec N
103 deg 05 min 30 sec E
