In the legends of different peoples, the story about the departure of the intelligent creatures that lived on it is widespread.
This applies to nagas, urags, vievichs and all other snake-people; rakshasas, hecatonheirers, fomorians and other many-armed and formless creatures; chuds, miniatures, gnomes and other dwarfs; Adityas, Daityas, Danavas, the Tribe of the goddess Danu / elves and other humanoid beings. They descended underground during catastrophes, which most often coincided with the defeat of their people in the war and ended with the destruction of the old world. A variant of this plot is the departure of many of the listed creatures into the sea, beyond the sea and into the depths of the sea.
In the Irish and Welsh sagas, as well as the Indian, Indo-Chinese and Polynesian epics, the close connection of the underground and underwater worlds is noted and it is said that one could get from one world to another along secret underground roads.
In legends, the underworld is considered a place of imprisonment of creatures who once lived on Earth - a hundred-handed hecatonheirers, titans, fallen angels, etc., then a fabulously rich country full of gold and precious stones, inhabited by nagas, vievichs and other serpent-people, and according to some legends also Daityas, Danavas and Elves.
According to the Indian epic, the underground country Patala with the capital Bhagavati is the most beautiful place on Earth, surpassing in beauty the "sky of Indra" - Svarga with the capital Amaravati. It has inner "suns", underground lakes, rivers and palaces made of pure crystal and other precious stones.
According to the Slavic, Nenets, Komi-Permian, Lapland (Sami), Scandinavian, Germanic, Altai, West African, Polynesian and other legends, the underworld served as the last resting place for pearls, chakli, gnomes, ieban and other dwarfs, whose main useful occupation was mining fossils.
The descriptions of the underworld in ancient Indian literature, Indo-Chinese legends, Irish and Welsh sagas, legends of the peoples of the North and other peoples allow us to trace the change in ideas about it depending on the time of creation of legends. At first, it was considered as a very real and attainable underground country or refuge, where gods or demigods and various demons descended or were cast down. Contacts between the inhabitants of the "lower" and "upper" worlds took place for a long time. Creatures living on the earth descended underground and stayed there, and the inhabitants of the underworld came to the surface and lived among the creatures inhabiting it. Often times, penetrations from one world to another ended in marriages between men from the “upper” world and women from the “lower” world, or vice versa. As a result, children were born who inhabited one of the worlds.
In the book "The Disappeared Inhabitants of the Earth" I explained why the ideas about the underworld and its inhabitants changed: after tens, hundreds of thousands or even millions of years, the compilers of the legends simply did not know what that world really was. Only vague echoes of very ancient legends about its existence reached them, so they endowed all the inhabitants of the "lower" world with ghostly, unnatural features, or made them something like the souls of deceased people and ghosts. And this, in turn, prompted many researchers of folklore to compare the underworld with the world of the dead or the dark kingdom of Hades.
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Consequently, the underworld as a country - at least in the form in which it is depicted in early legends - could not be the world of the dead. It was a world inhabited by creatures of flesh and blood. They were forced to go down there whenever there were global wars and disasters on the planet. After all, they had little choice - to fly into space, go under water or build shelters underground. And in most cases, the indigenous inhabitants of our planet, closely connected with the earth and, perhaps, simply did not have a sufficient number of space and underwater vehicles, did the latter.
You can ask me: is there at least some confirmation of all of the above. After all, if there have already been more than a dozen catastrophes on Earth, the earth's crust should be dotted with artificial underground cavities and be a kind of "sieve".
My answer is yes. Sources of information on this score are numerous publications by different authors from ancient times to the present day, teachings of secret schools and stories of researchers and travelers. All of them confirm that an extensive system of interconnected underground tunnels pierces our entire planet.