Salt production via boreholes was once prevalent in the entire Sichuan area - but for several factors (war, rebellion, taxes, wells drying up) Zigong became the center of production. Salt wells in Zigong were deeper and had better brine salinity - making for better yields in the salt extraction process. The method in which boreholes were drilled is significant. In 1835, the world's (at that time) deepest well was drilled reaching 3,300 feet. The miners of Zigong refined the techniques of deep borehole drilling. Later, Europeans copied and further refined the percussion drilling methods - so its effects can be seen in the modern Drilling rig used for oil and water.