In theory anyway, if we can find clear distinctions between natural vs. artificially altered cave entrances on earth, we can apply that to the moon and Mars to help us determine the likelihood of any cave entrances there being likely artificially altered than entirely natural.
In other words, we're looking for tell-tale signs of artificiality by the shape of the cave entrance.
So, the question is whether, for instance, cave dwelling entrances tend to be more regularly shaped - more likely to be a simple arch - or geometrically squared on sides or top - while naturally occurring - unaltered caves tend to be more irregular in shape.
Maybe the theory will not hold true - we'd have to test it once we have a sufficiently large collection of images of earth caves, or maybe with enough samples we could develop another way to determine which cave is more likely artificial by some other criteria.
If true, perhaps this theory could also be applied to smaller rocks such as we see in goggog's thread - although we'd also need a large sample of small rocks for comparison also to determine if there is a natural process which creates artificial looking entrances.
I would be willing to bet that NASA has already done a similar study. Take a look at the cave entrance they chose to land near on the moon for the Apollo 17 mission. The story is at this link:
Yes, good idea but what could we find out from those? ...And at least we can physically go to the places on Earth rather than relying on some shoddy images.
After thinking about this a little more, it occurs to me that it may behoove us to do a study of naturally-formed vs. artificially-formed cave openings on Earth.
Scientists are not always correct about cubic shaped rocks where there has been human intervention. Take for example the stones on the Egyptian pyramids. Cut from sandstone - or pounded from an aggregate?
This article is very deceptive, though much of it is correct.
It is entitled "Natural Formations that Look Man-Made." Therefore, everyone perceives that the article will only show formations that are entirely natural. However, that isn't the case.
For instance, the article leads one to believe that the square hole in the fairy chimney was made by natural geology. It shows you the picture, and then says:
"Vastly extreme landscapes exist in central Turkey. Magical fairies are said to have created extraordinary scenes. A thick layer of volcanic rock and ash helps to protect these beautiful stone formations. Over eons of time, the basalt cracks and the much softer inner stone begins to erode and wash away. This “Fairy Chimney” appears to have a window overlooking an enchanted land. Nature added a rainbow in this magnificent view."
However, re-read that paragraph. Notice that the square hole is never mentioned.
Later, at the bottom of the page, it adds more information:
"In Cappadocia, Turkey, eons of time have eroded jutting volcanic rock formations to create mystical Fairy Chimneys for as far as the human eye can see. The outer hard layer of stone is less easily-eroded than the inner soft sedimentary rock. Primitive people carved out homes, chapels, and tombs in the stone. This territory in Turkey is epically strange yet enchanting. This mystical and peculiar landscape continues below ground where the rocks have been tunneled out to create underground and otherworldly cities."
In fact, primitive people did carve homes into these volcanic chimneys - and that would have included square shaped windows and arched doors. In fact, Turkey is one area of the world where we find cuts into cliff walls - man-made cave entrances all over the place.
See more of such cuts into the volcanic chimneys in this video:
Later, we see the Indian face in Colorado. Again, is it man-made or natural or a combination? The article leads us to believe it is entirely natural (and it may be)- but yet there is no evidence of this, nor is any study of the same mentioned in the article. Wouldn't you like to know how they determine a thing is entirely natural?
Later we also have the cubic-shaped blocks in Peru. The article says:
"In Peru, these stone formations were once thought to be ruins from an ancient “Lost City.” Archaeologists and geological analysis proved these rectangular rocks are a natural design. Each architectural piece is made of sandstone that showed no signs of intervention by the hands of man. Instead of ruins from the Inca people, nature once again created rock formations that look like they were made by man. (bottom left)"
Again, "no signs of intervention" were found. These are sandstone objects. How much weathering may have removed any such signs on such soft stones? What if they are extremely old - or if some better developed technology created them - i.e. - what if they were carved with water and not stone tools? The article suggests that because we have found no signs of human intervention they must have been created by nature - but fail to show how nature created them.
The problem with this type of article is that it presents a straw man - where people believe that these things have been established - when in fact, they are speculating or even misleading about some of the items pictured.
On another forum, during the course of a 'discussion' about proof for aliens someone offered this page on natural rock formations and rocky outcrops which look like they are man-made but in fact they are natural. (as far as we can tell I suppose)
Some are quite interesting - the square hole in the cliff face, for example.