The Book of Mountains and Seas also mentions Kunlun Mountain, which is a wonderful book in ancient China. The book records a lot of things that we can't explain scientifically now. So this is a magical book, so the Kunlun Mountain recorded in the book is also amazing.
Because a legend is true, it is said that many expeditions have been to Kunlun Mountain, but their deaths are very strange, so Kunlun Mountain is forbidden to enter and is guarded by troops. Many people think that Kunlun Mountain may be similar to the matters recorded in myths and legends, and it is a place where people and gods live together.
It's just that human wisdom is developing too fast, and human beings are very clever creatures, so the immortals feel threatened. In order to keep secret, they drove all the officials away and didn't let anyone near this place.
It may belong to a very high altitude area, or it may be surrounded by fairy fog, which makes people feel like they are in a fairyland. We can see it in various martial arts novels or some TV dramas that cultivate immortals, which makes people feel that only immortals can live in such a place.
Just to make it immortal, let people think that Kunlun Mountain is not on earth, but suspended in mid-air, which is inaccessible to ordinary people, just like the worse gate leading to heaven, belonging to the air.