The U.S. has agreed to sell South Korea GBU-28 bunker buster bombs. This 19 foot long, 4700 pound weapon was designed and built in 17 days during the 1991 Gulf War. It can penetrate more than 20 feet of reinforced concrete or 100 ft of earth, before detonating 675 pounds of Tritonal explosive inside a bunker. The bomb is actually a BLU-113 fitted with a laser guidance kit, turning it into the GBU-28.
Saddam Hussein was known to have built two heavily protected underground shelters in 1982 and 1983. The German firms that designed and constructed these 20,000 square foot shelters described them as having 7 feet of reinforced concrete for a ceiling and walls of five feet of reinforced concrete. The main entrance was through a three ton steel door. There were two escape tunnels, one going 200 meters to the Tigris river. One bunker was under a Presidential Palace, which would have to be leveled before the GBU-28 could get a shot at the bunker several dozen feet underneath.
North Korea has long been installing military installations in large underground bunkers, often dug into the sides of mountains. North Korea has lots of mountains for this. North Korea has become so good at this, that they sell their expertise to other countries. Iran is a current client.
South Korea is buying large quantities of American smart bombs, to be used by their new F-15 fighter-bombers.