Mining metal and minerals in ‘ecosystems we hardly understand’ poses grave threat to marine life, warn environmentalists. Suzanne Goldenberg reports This is the last frontier: the ocean floor, 4,000 metres beneath the waters of the central Pacific, where mining companies are now exploring for the rich deposits of ores needed to keep industry humming and smartphones switched on. The prospectof a race to the bottom of the ocean – a 21st-century high seas version of the Klondike gold rush – has alarmed scientists.