Engineering the Impossible.
One problem the US faced consistently with Basement operations is mobility. On top of traps, various chasms, water hazards, pits and other environmental problems too numerous to mention. Thus the Delver Combat Engineering unit was formed, with the unenviable task of taming an environment they had no hope of taming. Even more then an average combat zone, engineering in the basement is a temporary measure.
It is one that the Engineers take to with gusto though. Gaps are covered with temporary bridges, pre-built stairs are set into place, the electrical systems are maintained, temporary forward bases are established, systems are built to haul larger finds out, all under the same constant threat the DOG's face.
For this reason the US controlled areas of the Basements have the most extensive human development. It is also not common to discover lost or forgotten US projects in other areas. Due to the capricious nature of the Basement.
The Engineers use the same weapons as their comrades but they have two speciality items that make life easier. The Military Mule and M-Gator are both light enough and small enough to move through most Basement Architectural Tropes. Though it is a slow experience as each inch must be cleared and checked for traps.