Sentence:  From his birch-paneled office in the Laboratory of Physiological Hygiene , under the university 's football stadium in Minneapolis ( `` We get a rumble on every touchdown '' ) , blocky , grey-haired Dr. Keys directs an ambitious , $ 200000 - a-year experiment on diet , which <WSD>spans</WSD> three continents and seven nations and is still growing . 
Meaning of the word marked with <WSD> tags:  to cover or extend over an area or time period