Sentence:  Buried within the Mediterranean littoral are some seventy to ninety million tons of slag from ancient smelting, about a third of it concentrated in Iberia. This ceaseless industrial fueling caused the deforestation of an estimated fifty to seventy million <WSD>acres</WSD> of woodlands. 
Meaning of the word marked with <WSD> tags:  An English unit of land area (symbol: '''a.''' or '''ac.''') originally denoting a day's plowing for a yoke of oxen, now standardized as 4,840 square yards or 4,046.86 square meters.