North Korea has dug a tunnel more than 500 metres deep at its nuclear test site in possible preparation for another test. A South Korean newspaper, on Wednesday said the work is going on in Punggye district in the northeastern province of North Hamkyong, the region where the North staged tests in 2006 and 2009. Chosun Ilbo, quoting intelligence sources, said the North is also reportedly speeding up major excavation work and construction of a new building at its main nuclear complex in Yongbyon, north of Pyongyang. It said North Korea is busy digging even in winter when the ground is frozen at two nuclear facilities in Punggye district and Yongbyon. Chosun Ilbo quoted an intelligence official as saying if progress goes on at the current pace, the North will have dug a cave one kilometre deep, the depth where it is possible to conduct a nuclear test, between March and May next year.