Dunno where that picture is from, it makes it look much more like a human construction, here's a more recent pic. If you want to learn more about this rock wall, here's a link.
TL;DR It's sandstone that fractured (something sandstone does all the time) but happened to fracture into a pattern that looks like stacked stones. This apparently happens a lot to rock that is underground and while there were legends of arches and hallways and even "metal tools" there is no actual evidence to say those existed and the "metal tools" where simply iron deposit in the earth which happens all the time.
