From Nothing to Envy: Ordinary Lives in North Korea:

Unlike the Chinese, the Mongolians allowed the South Korean embassy in Ulaanbaatar, the Mongolian capital, to accept North Korea defectors.  In fact, if North Koreans managed to sneak across the Chinese border into Mongolia, they would be arrested by Mongolian border police and turned over to be deported – to South Korea.  Getting arrested in Mongolia was in essence a free plane ticket to Seoul.  As a result, Mongolia had become a major depot on what had become a veritable underground railroad ushering North Koreans to South Korea.

But as you’d expect, this underground railroad can be tragically deadly:

The Gobi Desert temperatures were soaring into the 90s… The six liters of water they’d [a band of North Korean defectors] brought were finished.  Hyuck and the others took turns carrying the three-year-old, but when the ten-year-old started flagging, they couldn’t do anything but drag him along.  They finally found an abandoned hut near a small pond.  One of the women stayed with the boy while Hyuck ran off to get water.  As he approached, he heard the woman screaming.  The boy was dead.