How do you get 20m tonnes of grain out of Ukraine?

Grain in a facility in the Donetsk region that was shelled amid Russia's invasion of Ukraine. Photograph: Serhii Nuzhnenko/Reuters

A clock is ticking inside the towering, multistorey warehouses on the quayside at the Black Sea port of Odesa on the Ukrainian coast.

One huge metal structure alone contains a quarter of a million tonnes of grain, yet represents just over 1% of the estimated 20m tonnes trapped in Ukraine since the Russian invasion began in late February.