In the era of the internet it has become pretty easy to create things by creating a homepage about them. This happens all the time and for various reasons. For example, there is the Hommingberger Gepardenforelle, invented by an German computer journal. They created a mystical trout to check how search engines would list the term.
Inventing a cave by creating a website is pretty strange. Not many people are interested in caves, most like them as a movie location, if the scene does not take too long. Creating a cave hoax for explaining the source of faked archaeological treasures makes some sense. But a virtual cave, which is only funny for people knowing more about caves, is a sort of insider joke. Both examples we found on the web are from Great Britain, and we guess it is an example of the proverbial British Humour.
So no matter how good those webisites look, this caves do not exist. Do not let them fool you!