Which feature is unique to underground tours as opposed to outdoor ones?

The exploration of deeper geological layers is a feature that is unique to underground tours. When visiting caves, one can observe various rock formations, mineral deposits, and the geological history embedded within the layers of the earth that are not accessible in outdoor environments. The stratification and composition of these layers provide insight into the geological processes that have occurred over thousands or millions of years, making underground tours a unique educational experience.

In contrast, the visibility of natural sunlight is a characteristic of outdoor tours, as these environments are exposed to the sun and do not provide the same immersive experience of darkness and light conditions found underground. Similarly, above-ground historical sites are typically limited to outdoor settings where the structures are on the surface. Lastly, outdoor weather conditions influence outdoor tours significantly, but are irrelevant in the controlled climate of underground tours, where weather does not play a role in the experience.