I watched recently the series about King Arthur. It is of course well known Legend, placing him in 5th century or similar. Now what paid my attention is his death. English newer version tells about Avalon Island. Welsh version tells, that he sleeps with Guinevere and his knights in the underground cave in Welsh. They were even searching him in the caves in series. 
There are similar legends.
Slovenia; King Mathias and his wife Alenka (Madeleine) were fighting with his knights terrible battles against invaders. When invaders became too strong, they found the shelter in the underground cave under the mountain, where they sleep. One day they will wake up and defeat the invaders... The legend is well spread in Slovenia, they tried to connect it with Hungarian 15th century king Matias Korvin. However the similarity are just the names.
Czechia; An ancient legend says that a large army of Czech knights led by St. Wenceslashttps://en.wikipedia.org/wiki/St._Wenceslas sleeps inside the mountain Blanik. The knights awaken to help the Motherland when it is in great danger. According to the legend, when this happens, Blaník's trees will dry out but an old, dead oakhttps://en.wikipedia.org/wiki/Oak tree under the mountain will turn green and a small spring by the mountain will become a river.
It seems that this legends have perhaps the same origin? In forgotten past long ago?
Are there any other similar legends, anyone knows perhaps?
There are similar legends.
Slovenia; King Mathias and his wife Alenka (Madeleine) were fighting with his knights terrible battles against invaders. When invaders became too strong, they found the shelter in the underground cave under the mountain, where they sleep. One day they will wake up and defeat the invaders... The legend is well spread in Slovenia, they tried to connect it with Hungarian 15th century king Matias Korvin. However the similarity are just the names.
Czechia; An ancient legend says that a large army of Czech knights led by St. Wenceslashttps://en.wikipedia.org/wiki/St._Wenceslas sleeps inside the mountain Blanik. The knights awaken to help the Motherland when it is in great danger. According to the legend, when this happens, Blaník's trees will dry out but an old, dead oakhttps://en.wikipedia.org/wiki/Oak tree under the mountain will turn green and a small spring by the mountain will become a river.
It seems that this legends have perhaps the same origin? In forgotten past long ago?
Are there any other similar legends, anyone knows perhaps?