Reserves

Reserves are the quantity of a mineral or fuel that geological and economic studies show can be profitably extracted with current technology and prices.

Reserves are not the same thing as the total amount of a mineral or fuel that exists underground; they are the portion that has been confirmed by drilling and testing and that can be extracted profitably at current prices and technology. A deposit can be enormous in the ground while its reserves, the economically recoverable share, are far smaller.

This is why reserve estimates change even when nothing has been discovered or removed: a rise in prices or an improvement in extraction technology can turn previously uneconomic material into reserves, and a price drop can do the reverse. Reserve figures reported by companies and governments are a snapshot of current economics and geology together, not a fixed measure of what is physically present, and they should not be read as an estimate of how long supply will last at current production rates.