The legend about the wizard Merlin and King Arthur contains numerous caves. One of them is the cave of the Wizard. Merlin has this mysterious hideout, where he learns, lives and does magic.
And it is the place where he is out of the normal time. Either the time goes different in his cave, or he sleeps for a very long time. Again a very common motive, Avalon, the magical island, is also told to have a different time axis. This motive is not really unique, as many fairy tales and legends tell about people who went into magic caves and after coming bach some minutes or hours later, they found that anybody they knew had died many years before.
Modern interpretations by Hollywood movies even tell about time dilatation during space flights close to light speed. Of course, a hijack by aliens would explain those stories.....
But back to Merlin's cave: he was trapped by his enemy, the witch Morgana, in his cave and so he was not able to help King Arthur and fight against his fate. And he is still a prisoner in this cave.