Who made them, why did they make them and how did they make them? We certainly can't duplicate them with modern technology, presuming that we wanted to. They have been found in various locations throughout the world. Microscopic examination of the "legitimate" crystal skulls shows no tool marks.
Scientists are finding it difficult to explain how a primitive culture (because the skulls are known to be at least several hundred years old and possibly some several thosusand) could have carved crystal into such precise shapes and against the natural axis of the crystal.
Crystal is a mineral characterized by perfect molecular symmetry . When a "jeweler" cuts crystal, and attempts to carve against this axis, the crystal shatters.
Even with today’s elaborate laser equipment, it would be very difficult to create such crystal sculptures.
500 Million Year Old Footprint in Rock
This fossilized sandal print is embedded in rock which is supposed to be from 300 million to 600 million years old. It's clear that the rock was formed After this print was made.
Now, either 1)This is a fake (its been tested and is not) 2)or--This rock is not really 300 million plus years old 3)or---the rock is 300 million plus years old and man was around 300 million years ago.
This artifact was found in Utah in the 1960's. It's no wonder that geologists and the like are not swarming over it because, whichever of the solutions you accept above (except that it's a fake), it obliterates a number of the tenets of modern dating, geology and evolutionary science.
This photo was taken in northern Washington state and was reportedly found with another partial imprint. It appears to be the shoeprint of a large individual (see man's shoe in lower left of photo for comparison)approximately 16 inches long from heel to toe.
The rock itself is judged by geologists in "evolutionary time" (as opposed to actual time) to be between 10 and 20 million years old. The point is, according to evolutionary theory, no one should have been around early enough to leave a shoe imprint in what is now solid rock. No one should have been around to draw one either.
There remains the possibilty of course that someone painstakingly chiseled out two convincing shoeprints in recent times and left them on a mountain.