Recent security breaches at Incheon International Airport, which saw three foreigners entering the country illegally and vanishing into the night, have exposed a much wider problem with immigration safeguards.

One common modus operandi of illegal immigrants is to arrive as tourists and then simply stay and look for work.

Jeju Island has become a main conduit for illegal entry after the island permitted 30-day visa-free entry to travelers from 192 countries in 2006. In 2000, more than 1,000 illegal aliens were caught trying to enter Korea by sea, but now the preferred route is by plane, a police officer said.

On Jan. 13, 59 Vietnamese tourists who came to Jeju as part of a tour group disappeared. Thirty-one are still unaccounted for. Between 2011 and 2015, some 7,187 tourists who entered Korea visa-free subsequently vanished. Investigators say they include around 3,000 Chinese who are feared to have slipped across to the mainland.