A Dark, Striated “What am I?” Quiz

A Dark, Striated “What am I?” Quiz

Nikon D4 + Venus 60mm Super Macro f/2.8 — ¹/125 sec, f/4, ISO 6400 — map & image data — nearby photos

What am I?

Time for another one of my “What am I?” quizzes. What's the thing above?

As usual, I'll keep answers private until I reveal the answer in a few days...