There are children who are begotten by two sorcerers or wizards in the spirit world; scientists killed artificially on earth and kept there to improve the technology in the underworld; and some skilful sorcerers, wizards and magicians who are able to go under water.
There are mermaids (female) or merman (male).
In every country in the world, you will find the same structures of human activities under the lakes and rivers, which affect the life of those on the earth above.
Underworld is under water.
A man married a mermaid.