Sentence:  The dawn of the oil age was fairly recent. Although the stuff was used to waterproof boats in the Middle East 6,000 years ago, extracting it in earnest began only in 1859 after an oil strike in Pennsylvania. The first barrels of <WSD>crude</WSD> fetched $18 (around $450 at today's prices). 
Meaning of the word marked with <WSD> tags:  Crude oil.