On Wednesday, March 6, 2019 at 4:59:22 PM UTC-8, RichA wrote:
Ever dig in soil and hit a rock? Seems NASA didn't expect it. Pretty stupid.
https://www.bbc.com/news/science-environment-47469071
From your own link, which you apparently did not read with full comprehension...
"... But the presence of hidden rocks was always a possibility, and even expected."
\Paul A