What term describes ducts that are installed beneath ground level?

The term that describes ducts installed beneath ground level is "Underground Ducts." This terminology is widely used in the mechanical and HVAC industries to specify ducts that are located below the surface. Such installations are common for various applications, including utility distribution for heating, ventilation, and air conditioning systems, as well as for the transportation of air or other materials.

Using the term "Underground Ducts" helps distinguish these ducts from those that are either indoor (in a building) or outdoor (exposed to the elements at ground level). Understanding the placement of these ducts is essential for proper installation and maintenance, as they often require different considerations for ventilation, drainage, and protection against environmental factors compared to ducts that are not buried.