North Korea and Iran have large underground facilties where they have their nuclear weapons and other operations. DARPA is looking at breakthroughs for dealing with tunnels and underground cities.
Underground is the new frontier for military and security action. Although tunnels were already extensively used in the Vietnam War in the 1960s and 1970s. Tunnels are more and more of a problem in Mexico and Middle East.
Vietnam now gives tourists visits to the tunnel complexes used in the Vietnam war.
North Korea had and probably has invasion tunnels into South Korea
Advances in robotics, autonomy, and even biological systems could rapid exploration and exploitation of underground environments that are too dangerous for humans.
The goal of the DARPA Subterranean Challenge is to discover innovative solutions to rapidly and remotely map, navigate, and search complex underground environments, including human-made tunnel systems, urban and municipal underground infrastructure, and/or natural cave networks. The representative backdrop for the competition events is a disaster-related search and rescue response to a developing and time-sensitive situation with an unknown layout. The DARPA Subterranean Challenge seeks to advance technologies through both Systems and Virtual competitions that will dramatically outperform current approaches of manually and laboriously mapping and searching subterranean environments.
The DARPA Subterranean Challenge envisions multi-disciplinary teams competing.
SOURCES- DARPA, Vietnam archive photos, Vietnam tourist photos