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 <WSD>strike</WSD> in Pennsylvania. The first barrels of crude fetched $18 (around $450 at today's prices). 
Meaning of the word marked with <WSD> tags:  The discovery of a source of something.