By Marcelo Rochabrun and Santiago Limachi CERRO RICO, Bolivia (Reuters) - Next to one of Bolivia s poorest cities lies one of the richest silver mines in the history of mankind - Cerro Rico, Spanish for Rich Mountain - a once-legendary source of wealth for the Spanish Empire that is now being dug out by artisanal miners. The miners - independent workers who labor in dangerous conditions in dark and poorly-ventilated tunnels - live off what they earn from the scraps of silver they find in Cerro Rico, which is slowly sinking and collapsing onto itself. Work is so fraught that many miners drink straight ethanol before going inside the tunnels, Reuters witnessed. They also chew coca leaves, a plant that has been used as an energy supply for centuries in the Andes and that is also the raw ingredient of cocaine. It s impossible to work without coca in the mine because of the toxic gases, said Miguel Angel Delgadillo, who has worked in Cerro Rico for 25 years. Coca leaves are chewed in the s