"Tunnel rats" are know from the war in Indochina. Later, American soldiers were forced to fight underground (literally) in the mountains (more precisely, caves) of Afghanistan. In preparation for future battles fought underground, they train hard in South Korea.
Soldiers of the 2nd Infantry Division, learnied to bypass or fool seismic sensors, cameras and other electronic traps.
There are four well-known tunnels in the demilitarized zone between the North and South Korea. It is not known how many such underground passages have not yet been discovered. South Korean sappers have built a training facility for combat training in the city, and a network of training tunnels has also been established within it.
In March this year, the international Warrior Shield exercise took place in Korea, in which soldiers practiced teamwork while storming buildings and tunnels underneath.
Source: Soldier Systems