Heap-leaching enables the economic extraction of gold from low-grade ores; some modern mines use as much as 30 tons of rock to produce a single ounce of precious metal. But this process can be highly damaging to the environment. Cyanide is one of the most potent poisons known; a pellet the size of a grain of rice can kill a person. Most spent cyanide solution is stored in reservoirs, where it gradually breaks down. But these reservoirs are prone to accidents. In 2000, at a gold mine in Romania operated by the Australian firm Esmeralda Exploration. 100000 tons of wastewater laced with cyanide spilled into a tributary of the Danube River. The toxic plume washed all the way to the black sea, causing a massive kill of fish and birds and contaminating the drinking water of 2.5 million people.