So far they have found four of them. There are many who believe there are dozens more tunnels under the DMZ, the De-Militarized Zone that straddles the 38th Parallel, the frontier dividing the two Koreas.
The tunnels were dug by the North Koreans. They were (are?) to be used to insert thousands of soldiers directly into the South in the event of war. The North claims it was coal-mining.
However the tunnels run North to South and there are no branches off them. Also they are dug through granite, not anthracite.
It is no wonder that there is an absence of trust in peace in South Korea. I was here two and a half years ago when the North put dozens of artillery shells into a South Korean island.
There was no response then from the government in Seoul. That was despite the fact that just a few months earlier a South Korean corvette had been sunk by a textbook torpedo strike with the loss of forty-six lives. The response then: also nothing.
But there is now a new Korean president and for her, it seems, inaction is not an option.
She has just warned the North that new rules of engagement mean immediate violent retaliation will follow direct provocation.
The North's response has been to announce the re-starting of the Yongbyon nuclear reactor.
It was mothballed in 2007 as part of an international agreement. Powering it up again is not a direct provocation, but it is adversarial.
There seems to be a daily drip drip to increase tension along the Korean Peninsula.