system
Given the following post, respond as if you're commenting on Reddit:
human
Are there ways to find caves with no real entrances and how common are these caves?

I just toured the Lewis and Clark Caverns today and it got me wondering about how many caves there must be on Earth that we don't know about simply because there is no entrance to them.  Is there a way we can detect these caves and if so, are there estimates for how many there are on Earth?
gpt
There are, many of them former from trapped gas pockets during volcanic eruptions, or groundwater eroding softer stone and then changes in flow leave the chamber intact but unreachable. 

There are several ways to find such voids, we can detect them by drilling when the bit drops unexpectedly, if they are close enough to the surface using ground penetrating radar, from orbit if they are large enough by temperature variances where none should be there, or using a technique to measure electrical resistance across a given section of dirt in areas with the right soil type and moisture content (the void space doesn't conduct electricity so the value is lower than a solid chunk)