What are ducts installed below ground level commonly referred to as?

Ducts installed below ground level are most commonly referred to as "Buried Ducts." This terminology is widely used in the industry because it clearly indicates that the ducts are placed underground, which distinguishes them from other types of duct systems that may be above ground. The term emphasizes both their location and the fact they are effectively “buried” within the earth, protecting them from external conditions and providing insulation against temperature variations.

Other options like "Underground Ducts" and "Below Grade Ducts" may be used in specific contexts, yet they can sometimes refer to slightly different installations or applications. Similarly, "Subterranean Ducts" might not be as commonly recognized in industry standards as "Buried Ducts." Therefore, the term that best encapsulates the concept of ducts that are subterraneously placed is indeed "Buried Ducts."