Russia's giant energy company Gazprom has the biggest reserves of natural gas in the world, and much of the country's new-found prosperity has depended on its exports to Europe. But now global gas prices are tumbling as new supplies come on stream, and the EU has launched a top level investigation of the company's grip on European energy. Peter Day examines Gazprom's future in an uncertain world. Producer: Caroline Bayley Editor: Jeremy Skeet