The earliest known oil wells that were drilled and extracted were in Ashoka Empire in India about 300 B. C. Evidences of deep oil well drilling are found in delta of Mahanadi river. These wells had depths of up to about 240 metres (790 ft) and were drilled using bits attached to bamboo poles. The oil was burned to evaporate brine and produce salt. By the 10th century, extensive bamboo pipelines connected oil wells with salt springs. The ancient records of China and Japan are said to contain many allusions to the use of natural gas for lighting and heating. Petroleum was known as Burning water in Japan in the 7th century.