What must be done to openings of abandoned mines?

Fencing off and posting signs at the openings of abandoned mines is a crucial safety measure. This action prevents unauthorized entry and helps to protect individuals from potentially hazardous environments. Abandoned mine sites can contain unstable structures, toxic gases, and other dangers that pose significant risks to people who might unknowingly wander into the area. By clearly marking these locations with appropriate signs and physical barriers, it ensures that individuals are aware of the hazards and discouraged from entering, thereby promoting public safety and minimizing liability for potential accidents associated with these sites.

The other options do not adequately address safety concerns. Marking for future excavation does not prevent access, filling openings with water could create additional hazards if not managed properly, and leaving them open for ventilation is counterintuitive since it would expose people to danger rather than protecting them.