Jellyfish are taking over the world's oceans, eating baby fish and driving marine ecosystems back to the primitive Cambrian era. Or are they? Although incidents of human-jellyfish interaction are on the increase, it's hard to be sure that the jellies are really increasing in number over the long term. But then again, if we wait till we are sure, won't it be too late? Miranda Krestovnikoff investigates.

Producer: Jolyon Jenkins.