What is one key characteristic that differentiates caves from other geological formations?

Caves are primarily defined by their underground nature, which sets them apart from other geological formations. This characteristic is integral to their formation, as caves are typically formed through processes involving erosion, dissolution of soluble rocks like limestone, or other geological activities that occur below the earth's surface.

Unlike formations that are predominantly visible above ground, such as mountains or cliffs, caves are concealed within the terrain, often requiring exploration and specific access points to enter. This unique underground aspect not only influences the cave's geological and ecological characteristics but also impacts the types of formations, such as stalactites and stalagmites, that can develop within them.

The other options do not encapsulate the essential distinction of caves. Above-ground visibility is a defining aspect of many geological formations and doesn't pertain specifically to caves. Unique color patterns might be a feature of some caves but are not a differentiator across all caves. Similarly, while some caves can form from volcanic activity, many others result from different geological processes, making it not a universal characteristic. Thus, the underground nature of caves is what truly defines them in comparison to other geological structures.