Friday Photo Quiz

I've been posting these on Facebook and despite their loose permissions policy, you still cannot view my profile unless you are a member, so...

Every Friday I will post a picture here that I've taken in my travels and ask you to identify something about it.

This week we go underground. Do you recognize this formation and where it is? Please do not give the answer but feel free to leave a comment that you do know. I'll post the answer on Sunday.