Sentence:  Buried within the Mediterranean littoral are some seventy to ninety million tons of slag from ancient smelting, about a third of it <WSD>concentrated</WSD> in Iberia. This ceaseless industrial fueling caused the deforestation of an estimated fifty to seventy million acres of woodlands. 
Meaning of the word marked with <WSD> tags:  To approach or meet in a common center; to consolidate.