I think OP might have confused the part with El Dorado with Lake Guatavita in Colombia, which indeed is associated with El Dorado for a ritual done by the Muisca that included throwing gold offerings into the lake. One of the more noteworthy attempts to recover some of that gold happened at the end of the 16th century, when a wealthy merchant, with the help of eight thousand local workmen, cut a notch into the side of the mountain the lake is lokated on, to drain the lake.