Operators returning to old ‘dry’ oil wells often find the black gold has returned—like magic. The explanation is that drilling recovers only about 30–40% of the oil that is actually there. That means 60% to 70% of the oil is being left behind in the rocks. This residual oil was already there, and some of it is later recovered through workovers, re-perforation, pressure support, steam, CO₂ or chemical injection. Higher prices or improved technology often make low-rate production economic again. So how big are the world’s known and unknown oil reserves? Are they vastly larger than the official figures shown below? * Proven oil reserves of about 1.65–1.7 trillion barrels — enough for roughly 50 years at today’s consumption. * Proven coal reserves are approximately 1.06 trillion tonnes — enough for roughly 130 years. * Proven natural gas reserves of around 7,300 trillion cubic feet — enough for well over a century in many estimates. What does this mean for the costly global transition to wind and solar energy? The $275 trillion figure comes from a McKinsey Global estimate of cumulative spending on physical assets for a net-zero pathway (roughly 2021–2050). The mystery in all this isn’t just about ‘endless oil’ or even its imminent exhaustion. It’s that we don’t know. – at Canberra, Australian Capital Territory