Alluvial diamond and gold mining activities also account for some economic activity. In recent years , foreign investment from ArcelorMittal Steel, BHP Biliton, and China Union is aiding the revitalization of the iron-ore mining sector. Liberia has begun exploration for offshore oil; unproven oil reserves may be in excess of one billion barrels. The government divided its offshore waters into 17 blocks and began auctioning off exploration licenses for the blocks in 2004, with further auctions in 2007 and 2009. An additional 13 ultra-deep offshore blocks were demarcated in 2011 and planned for auction. Among the companies to have won licenses are Repsol, Chevron, Anadarko and Woodside Petroleum.
Question: How many years spanned from 2004 to the last auction?
Answer: 6