As the worst smog in years smeared China's skies, a city is looking deep into the ground for ways to clear its air.

Baoding, in Hebei province, has vowed to tap its rich geothermal resources to free the city from smog.

Four large geothermal fields have been found in the city, covering a total area of 3,700 sq km. Most of the city has used the clean energy for heating and bathing.

Households using a geothermal heating system have increased rapidly, accounting for 90 percent in Xiongxian county since 1973, when the resource was first explored.

In addition, local hotels and spas offering hot springs are adding 200 million yuan ($32 million) every year to local revenue, due to the quality of their resources and good location, around 100 km from Beijing and Tianjin.

"The utilization of geothermal resources has never been more important," said Ma Yufeng, the city's mayor, adding that the city has to protect the environment and provide resources such as agricultural products for the capital because of its location.

"We also are pursuing the development of the economy, which means that further exploration of this clean resource will be one of our priorities," the mayor said.

A worker at Sinopec Green Energy Geothermal Development carries out equipment repair work to the geothermal heating system in Xiongxian county, Baoding, Hebei province in January, 2010. [HU QINGMING / FOR CHINA DAILY]