(geology) inner core

inside, within (physical location) / between, among / while, during, inside or within a span of time / (when used possessively) one's own, my, our

sooner or later / eventually / before long

nickel (element)

configuration, structure, construct, constitution

compose; constitute / configure

(astronomy, mythology) Earth; Terra

atomic nucleus / (algebra) kernel / (astronomy) core, nucleus / cell nucleus

(archaic) iron / (figuratively) something extremely strong

mainly, primarily