The much-scrutinized base would be an ideal spot for North Korea to set off an underground nuclear blast if it wanted to make a splash.

Since 1963, the site has been used only for underground blasts, and it is now the country's only nuclear bombing range.

Four days after the huge underground blast, the work has become sifting through the evidence, separating the dead ends from the fragmentary leads.

The network's origins go back to the cold war when nations detonated thousands of nuclear warheads, mainly in underground blasts.

In Switzerland, the majority of homes have an underground blast or fallout shelter.

In May, India and Pakistan stunned the world by conducting a series of underground nuclear blasts.

"There would have to be an excavation of some sort in order to set off an underground blast."

Wes says they've found no evidence to support the theory of man-produced underground blasts.

The official news agency Tass, which issued a two-paragraph report on the test, said the yield of the underground blast was less than 20 kilotons.

Yet some analysts said political foes of underground blasts might succeed in blocking the project.