What does an overcast in a mine do?

An overcast in a mine is specifically designed to facilitate the crossing of two air currents without allowing them to mix. This is critical in mining operations, as maintaining separate air currents helps control the flow of air within the mine, ensuring that fresh air can reach working areas while potentially hazardous gases or dust are kept separate. In some cases, this separation can also help manage temperature and humidity levels in different parts of the mine, ultimately supporting a safer working environment for miners.

The other options do not accurately describe the function of an overcast. For instance, while the structure may aid safe crossing for workers, this is not its primary purpose. Additionally, an overcast does not directly ventilate the mine in a general sense, and while it might restrict access to dangerous areas, that is not its intended function either. Instead, it serves chiefly to manage air currents effectively.