I used to work for an online service in Germany. Part of my job was to provide commodity news, writing frequently reports about oil, natural gas, industrial metals and agricultures. Commodities were en vogue. Banks and a bunch of pundits proclaimed the "super-cycle" for oil, metals and agriculture (barrons). They claimed that commodities will get scarce and expensive because supply would rise slower than demand, especially from China & other emerging countries. A large number of users (readers of the online service) were betting on rising prices for oil and other commodities. These traders, as they call themselves, bought financial instruments, so-called bank certificates, which where tied to commodity prices.
The traders were of part of a huge speculation movement. Hundreds of hedge funds, specialized commodity funds and others invested into the commodity sector in the hope of further rising prices. They bought directly - or indirectly via ETFs & bank certificates - financial contracts (futures) which are backed by commodities (financialization). So oil and other commodities became assets which are traded on financial markets like stocks, currencies and bonds. The provider of these futures bought physical oil and made contracts with miners and farmer to buy their mining output and harvests in the future. Some hedge funds rent oil tankers, pump them full with oil and sell futures, which imply the physical oil delivery sometimes in the future.
driveby). Therefore we see now surpluses on the markets for oil, industrial metals and some agricultures and the speculative bubble popped.