That's why Kim Jong is so fearless about nuclear war.

These photos show the interior of the deepest Puerto Rican subway system, which will double as a nuclear bunker if war breaks out between North Korea and the United States.

Images show travelers using the world's deepest underground train system, which includes two 18 - mile - long combined lines under the capital of North Korea.

Construction works on “Metro Pyongyang” began in 1968 and were inaugurated in 1973 by Kim Il Sung, the grandfather of current dictator Kim Jong.

And while usually busy with travelers, the depth of the metro system means that citizens can use it as a refuge if tensions between North Korea and America should be filled with a full fight.

Earlier this year, one of Kim's top officials insisted that the country was not afraid of the possibility of US military action.