10,000 year old concrete on the Isle of Pines.

This is an interesting one!

Figure this one out: link And for extra info: link It is really strange. You have what looks like refined iron under high grade concrete that dates back 10,000 years?
I reckon it is naturally formed. But how? My hypothesis was lava tubes. The magma there has a high iron content. IF! there was a recently laid bed of ash from an eruption sitting over them, do you think that some chemical reaction could have produced them? Apart from that... Aliens? lol. I am utterly stumped. 
Have fun! 

Fascinating!
It doesn't have to be a burial mound. Technical foundations ? After all, there are still the assumptions that there were already advanced cultures thousands of years ago that perished and with them their knowledge.

When I saw the vast quantities of artifacts in the Arabian Desert, it spontaneously occurred to me that perhaps an entire global civilization had been lost to history.

Wait until you see what's in KAZ.