Quirks Question: How much oil have we taken out of the ground?

How much oil have we removed from the ground, and what's taken its place?  According to Astrid Arts, a staff geologist with Cenovus Energy in Calgary, we've probably removed about a trillion barrels of oil from the Earth since we started extracting it commercially in the 19th century.  That's about six million Olympic-sized swimming pools.  But that doesn't leave large voids underground - oil is usually found in "spongy" rock or porous sand deposits. When oil is removed it leaves small gaps which are filled with water or gas.  There is a pressure reduction in the area from which oil or gas has been removed, and on rare occasions this has led to land subsidence or even small earthquakes.