The North Korean leader, Kim Jong Il, has a network of 300 metre (1,000 ft) deep emergency escape tunnels connecting Pyongyang with key sites around the country, a top-level defector claims.
The tunnels, reminiscent of the lair of a James Bond villain, are reported to contain railway lines, a water supply and even vegetation. According to Hwang Jang Yop, formerly North Korea's chief political philosopher, they connect areas as far as 30 miles (50km) away from Pyongyang, enabling the country's leaders to escape the capital and ultimately sail to China via a West coast port.
"There were fresh water and grass growing within an underground tunnel that linked Pyongyang to a nearby mountain," Mr Hwang told Radio Free Asia, a station supported by the US Government, which broadcasts into North Korea for three hours a day. "In particular, an ultra-deep underground tunnel was built to connect one of Kim's residences in Pyongyang to [the then port city of] Nampo."
Mr Hwang said that the tunnels also connect to Yeongwon, the site of a mountain villa, where Kim Il Sung, Mr Kim's father and North Korea's founding president, died of a heart attack in 1994. They also travel to Suncheon, north of Pyongyang, which is reported to be the site of a uranium mine.
During the 1950-53 Korean War, North Korea suffered intense American bombing raids. Since then, it has hidden many of its military assets in vast underground bases, which include missile silos, aircraft hangars, and naval ports, according to the South Korean government and US military.
Pyongyang's civilian subway system is itself 150 metres deep, no doubt because it would double as a civilian bomb shelter in the event of a war. During the 1970s, Pyongyang constructed infiltration tunnels into South Korea beneath the heavily defended border intended as a conduit for the spies and commandos of an underground invasion force. Twenty of these have been discovered and sealed off, and two of them serve as tourist attractions for curious coach parties.
Mr Hwang defected to South Korea while visiting Beijing in 1997 but it is the first time he has spoken publicly of the existence of Mr Kim's escape tunnels through which he claims to have travelled himself. He was formerly the chief intellectual behind the theory of "juche", the North Korean doctrine of "self-reliance" which forms the basis of the personality cult created around Mr Kim and his father, the "Dear Leader" and "Great Leader".
Today President Obama's envoy to North Korea, Stephen Bosworth, arrived in Pyongyang, the highest level US visitor since the then secretary of state, Madeleine Albright, met Mr Kim in 2000. His mission is to revive the long-running and inconclusive talks on North Korean nuclear disarmament hosted by the Government of China.
After reaching provisional agreement in 2007, North Korea angrily declared them dead in April after the United Nations Security Council denounced its firing of a long range rocket. This was followed in May by an underground nuclear test, and by the test firing of conventional short rage missiles.
There was speculation that the atmosphere of a confrontation was a blind to disguise North Korean weakness, after Mr Kim suffered a serious illness, probably a stroke in summer 2008. Photographs in the official media suggested that he has made at least a partial recovery, although he is not expected to meet Mr Bosworth.
Mr Bosworth's three-day visit brings a definitive end the policy pursued during most of the Bush Administration of holding North Korea at arm's length. He will be seeking to encourage the mild warming of relations which followed the chill in the first half of the year. In October, Mr Kim reportedly told the visiting Chinese Prime Minister, Wen Jiabao, that he would be willing to return to multilateral negotiations in the so-called Six Party Talks if the US would also engage in one-to-one talks.
But the bigger question is whether, having acquired and tested a small arsenal of nuclear warheads, the North will ever genuinely give up the status and deterrent power which they bring.