Bubbling Up

Rule of Thumb #473:  Up is always the direction bubbles run, certain conditions excepted.  I don't know how, exactly, these "bubbles" formed, but they appeared on a large piece of 3/16" (nearly 5 mm) thick steel plate.  In fact, there are lots of mysteries about this object, like why did the normally black carbon coating turn blue in places and what's the white outline around each of the "bubbles."  But whatever the answers, it's a naturally beautiful thing.