A hoax is a very old thing, people make things up. They do it to fool others, because they think it is funny, or to betray somebody. The number of hoaxes exploded with the internet, but obviously they were not invented by the internet. The first flood of hoaxes was in the Middle Ages, when the remains of Saints were produced and sold in huge amounts. Probably 99% of all relics are faked. We do not know much about Stone Age hoaxes, but we can suppose there were some. So do not believe all cave paintings' interpretation by honest scientsts, probably they were fooled by a hoax!
There have been several hoaxes related to caves during the last century. However, in all cases the caves existed, just the story connected with the cave was a hoax. We will tell you some of them.
And there are other invented underground structures, mines (Treacle Mines), hideouts (like Bin Laden's Cave), and of course all the treasures in mines, graves, caves, and catacombs.