Pre-historic civilization’s buildings?
Gotta be. Prolly made of Kryptonite.
http://seattletimes.com/html/localnews/2022578298_berthaessaysxml.html
One of the local school’s 5th graders are trying to figure it out, and looking at the history of the area. Lots of guesses of trains, etc. that are remnants of old Seattle that ARE buried under the existing surface. But Bertha is 80 feet deep or so, and the old Seattle surface is only down 20 feet or so IIRC. Perhaps to 50 feet in the old bay, but I think Bertha is inland from that now. I think it is in 15,000 year old glacial material.
On the 6-borings spaced 5-feet apart though, that only covers 25 feet wide. Bertha is 58-feet diameter, so seems they have left plenty of room to still miss something big.