Cave within titled property is still owned by the State; there must be permit before collecting plants and mineral deposit

Dear PAO,

My grandfather owned a property in the province. It is a titled land, but for the longest time, no one in the family had the time to manage it because my father had already passed away, so my mother and I moved here in Manila to live with my maternal relatives, while my uncle, my father's only sibling, used to work abroad. He decided to come back and settle in our province. He wants to manage the property, but he was surprised to learn that there is a natural cave within my grandfather's property. Is that our family's property as well? Can we collect the plants and mineral deposits that we can find there?