What type of ventilation system relies on natural air movement rather than mechanical means?

Gravity ventilation is the correct answer as it refers to a system that relies on natural air movement caused by density differences, typically due to temperature variations between indoor and outdoor air. This system functions based on the principles of convection, where warmer air, being less dense, rises and is displaced by cooler, denser air. In mining environments, gravity ventilation is often utilized when mechanical means are not available or practical, allowing for natural airflow to help with air quality and temperature control.

The other options involve mechanical methods to control air movement. Forced ventilation uses fans or blowers to create airflow, while both negative and positive pressure ventilation rely on creating a pressure difference to move air; in negative pressure ventilation, air is pulled out of a space, while in positive pressure ventilation, air is pushed into a space. These methods are crucial for ensuring adequate ventilation in underground mines but do not pertain to natural air movement.