I touched upon it in my other (reply)[https://www.reddit.com/r/newhampshire/comments/477ilc/abandonedinteresting_places_to_visit_in_nh/d0cv22d] to you but it's a matter of population and conservation.

If the year is 1970 and the Pumpelly Cave exists. It's not on any official maps and you're not going to see anything in a newspaper or hear about it on the radio, it's too niche. It then follows that the only way you will hear about it is because you are already interested in the mountain begin with.

You hike the mountain regularly and you make friends with a group that hikes there. After a while you get to know them better they tell you about the cave. They bring you out there sometime to find it.

Another scenario: You're incredibly interested in the culture and history behind the mountain so you read every book you can find in your local library. You find a book like Chamberlains and read up on the existence of the cave. You go out and find it.

Scenarios like these reduce the population that goes looking for something like this to a much smaller circle of people. The problem is we live in the information age, information is so easily accessible that when it comes to getting outdoors it displaces populations unnaturally. People are naturally drawn to that which is more popular among others, hell it's how this entire website works.

Posting about the Pumpelly Cave online as a mecca for all the bushwhackers, treasure hunters and explorers in New England (and beyond) is the equivalent to a Reddit hug of death. Too many people drawn to one location that would have otherwise never even heard of it causes unnatural stress. So to answer your question, this is one way in which things get destroyed.