depleted compartment

low-valent metals constitute Earth's core
what's soluble in them's rare in the crust
the steel our industries refine from ore's
sequestered age-long in the form of rust
what had been ferrous crashed out of the sea
when photosynthesis freed oxygen
a brave new world of biochemistry
that led--in time--to vertebrates and men
who now wage wars for tantalum and gold
as if they were more precious than the air
we risk our lives to delve them from the mould
'though planet-wide, they aren't all that rare
siderophiles are secrets the Earth keeps
until we learn to plumb the utmost deeps