The world's overflowing oil reserves?

Oil is making headlines again.
Never far from newsworthy, such is its over-riding economic and political importance, the spotlight has returned as harsh as ever.
The price of benchmark US light sweet crude recently reached its highest level for 13 years, and the cartel of petroleum exporting nations, Opec, maintained its decision to cut production - potentially fuelling yet more rises.
Add increasing violence in Iraq delaying its return to full oil production, and it is understandable that oil analysts have been getting the jitters.
Not to forget President George W Bush, who in an election year really does not want to see car-loving Americas moaning about high gasoline prices, which have also risen as a consequence.
And oil giant Shell already admitting twice this year that it has over-estimated its own oil reserves.
But looking behind the politics and other reasons for the upward pressure on prices (such as historically low oil and petrol stockpiles in the US), just how much oil is left under the surface of Planet Earth?