In an idyllic, crystal clear patch of the Pacific, locals are learning what goes down must come up - and they're told the consequences will be devastating. Lurking at the bottom of a giant blue lagoon - a pinprick in the enormous ocean expanse - is a huge, ticking, time bomb threatening very soon to disgorge tens of millions of litres of thick, bunker oil, destroying the environment and a fragile island economy. The recent Pacific tsunami threatened to unleash the black tide but - thankfully - it passed without incident. But very soon something's going to give.