The mid-ocean ridges where tectonic plates meet in the Atlantic and the Pacific Oceans are seething with volcanic activity. The most active areas are deep under water, several thousand metres down. Ocean water penetrates several kilometres down towards the centre of the Earth where the crust is fractured. Geologists call this hydrothermal activity. Liquid magma heats the water to about 400 °C before the water squirts back out again as an underwater geyser. The ocean water draws minerals and metals out of the Earth’s crust and carries these back up to the seabed. Gold, silver, copper, cobalt, zinc, and lead are all deposited when the hot springs meet the cold ocean water....