Technically, it has grains. As it hardens, the liquid steel stuff clumps together not too unlike bread, with the boundary and grain size / mix being factors that affect the quality of the steel.
More like cast iron, and some lower-grade steels. Higher-grade steels tend to have their pores closed off, the grains welded shut/together as a part of the mechanical processing.
Sorry, not being technical. The pits is probably a better way to put it. Point being just because the tank is empty doesn’t mean it’s safe. Should still fill with water.