${\mathrm{Throughout\ history,\ the\ search\ for\ salt\ has\ played\ an\ important\ role\ in\ society.\ Where\ it\ was\ scarce,\ salt\ was\ traded\ ounce\ for\ ounce\ with\ gold.\ Rome’s\ major\ highway\ was\ called\ the\ via\ salaria,\ that\ is\ the\ Salt\ road.\ Along\ the\ road,\ Roman\ soldiers\ transported\ salt\ crystals\ from\ the\ salt\ flats\ at\ Ostia\ up\ the\ Tiber\ River.\ In\ return,\ they\ received\ a\ salarium\ or\ salary,\ which\ was\ literally\ money\ paid\ to\ soldiers\ to\ buy\ salt.\ The\ old\ saying\ ‘worth\ their\ salt’.\ Which\ mean\ to\ be\ valuable\ derives\ from\ the\ custom\ of\ payment\ during\ the\ Empire.}}$ $$ ${\mathrm{What\ does\ the\ passage\ mainly\ discuss?}}$