Part of the Underground expansion for The Division, Underground Operations are the replayable missions set in the subway tunnels and underground spaces of the city.
Access to the missions is via the TOC found down the stairs from The Terminal social hub in the Base of Operations. Once you have access to the area (there is a small set up mission to do first) you will be able to 'plan' missions and activate them to play from the central desk. The operations are of different lengths and difficulties that you decide when setting them up. although you will need to do short (single Phase) missions first to unlock longer ones.
Each mission will take you into various parts of the Underground areas in the hunt for 1 or more gangs and their leaders. This is much like the above ground missions but the new environments are distinctive.
Once you have set up the require mission you can pass through the subway station entrance to reach the train to launch it. You will be transported to and from missions via a subway cutscene
No Respawn
All missions done in the Underground areas are in a no respawn zone. This means that if your squad is wiped then you will have to reset and restart the mission from the TOC.
Normal or hard missions are suited for solo players, while the Challenging and new Heroic mode missions will probably require a good group.