Since various space programs and their subsequent mass analysis of Phobos have determined there are large voids inside the moon, it cannot be natural - so plenty of time to explore the inside remains. Seems to be several large geometrical voids under all those right angle redundant surface striations.
Too bad the images posted are so washed out, hiding the noise reduced detail available in other images.
I'm pretty sure there are valid explanations for a natural origin for these voids.
Gotta run now. I'll look into it later.