Seoul , South Korea ( CNN ) -- Gen. Hahn Sung-Chu never believed North Korea could dig a tunnel that reached Seoul -- until now . Standing inside a basement of an apartment block in the heart of the capital , the former two-star general in the South Korean military says , `` This is a kind of invasion , North Korean soldiers working underneath us . '' Hahn says residents had complained of underground vibrations , but the subway does not run beneath them . He says dowsers detected three tunnels , 13 to 16 feet ( 4 to 5 meters ) wide at a depth of up to 39 feet ( 12 meters ) . His team drilled two bore holes to lower a camera , but before they could , they detected two underground explosions and their drill holes were blocked . Hahn is certain that North Korean soldiers were working beneath their feet , protecting the tunnel . North Korea upgrades space center for larger rockets . A history of tunnels . Four tunnels from the North have been found in all , although none since 1990 . The South Korean Defense Ministry still officially looks for them as it believes there may be 20 in all , but the budget is small and tunnel hunters believe it is merely a token effort . North Korea has said the tunnels were not for invasion , but part of its mining industry . While the Defense Ministry believes there may be up to several tunnels dug under the Demilitarized Zone , it is convinced that none would reach as far as Seoul . It believes that Pyongyang would not be able to dig more than 6 miles ( 10 kilometers ) from the DMZ -- the heavily fortified border -- because of the Imjin River . Seoul 's northwestern boundary is about 25 miles ( 40 kilometers ) from the DMZ . `` To dig tunnels tens of kilometers , it must be angled properly , '' says Defense Ministry spokesman Kim Min-Seok . `` There is a huge amount of groundwater in the Korean Peninsula , so water and soil have to be removed consistently . South Korea and the U.S. have always taken aerial photographs , and we found no evidence of this . '' A view from a former North Korean official . But a former intelligence official from North Korea says the notion is not as far-fetched as it might sound . The military defector , who is not using his name as he still has family in the North , said North Koreans `` usually pulled out soil and stones during the night so as not to be detected by the U.S. or South Korea . They dig into the ground vertically from 100 to 150 meters ( 328 to 492 feet ) and slope upwards to South Korea so the water drains back to the North . '' The man says he had knowledge of the tunnel digging when he lived in the North . Saying that operations peaked in the 1980s , he believes Pyongyang would still protect the multiple tunnels it dug over many decades . `` I was told the tunnels are not directly connected to the streets of Seoul because of the risk of being detected . The tunnels are connected to the sewers linked to the relevant organizations . '' The relevant organizations that would be targeted are the U.S. Embassy , the Blue House presidential compound and government buildings , he says . The former official says that in case of war , infantry troops would flood into the tunnels dressed in American or South Korean uniforms . The tunnels would then be blown up behind them so there would be no retreat . But in recent years , many suspected tunnels have been proved false . And concerns about Pyongyang have shifted to its nuclear ambitions and long-range ballistic missiles . Tunnel hunters do not always get much respect in South Korea . But Hahn , and others who have dedicated their time and money to the search , say the government should not ignore the threat they believe exists under their very feet .
Ex - S. Korean general suspects a North Korean tunnel is under a Seoul apartment complex . But the Korean Defense Ministry believes tunnels ca n't exist beyond 6 km from the city . A source says tunnel operations began in the 1980s .