What does the term "hypogeum" refer to in relation to Roman architecture?

The term "hypogeum" in relation to Roman architecture specifically refers to an underground area beneath structures. This term typically designates spaces that are located below ground, and in the context of Roman architecture, it is often associated with various uses, such as tombs, storage areas, or even complex systems beneath theaters and arenas. Such underground levels could serve practical purposes, including the management of utilities, ventilation, or the housing of essential infrastructure. The concept of the hypogeum was crucial in Roman engineering, allowing for more intricate designs and functionalities within public and private buildings. This underground aspect is distinctly different from the other options, which focus on above-ground features or decorative elements.