Novel about underground species of humans, and a historical Satan

Question

I read this book some time in the 1990s. It was set during the near future. The plot revolved around the discovery of a species of humans who had established a civilization underground, but they were in decline. They occasionally made raids where they would capture normal humans from the surface for enslavement, and sometimes for breeding. They were known for savagery, and one of the artefacts that they had produced was a "beach ball" made of human skin of different races.

They also had the ability to preserve their consciousness during death and pass it to another living human. It was suggested Buddhist leaders who reincarnate, such as the Dalai Lama, were doing a similar thing.

An expedition was sent underground, during which a distress message was received from a Chinese submarine which had been sent some time in the future, and it was revealed that the leader of the subterranean humans had lived for thousands of years and was actually the historical basis for the Satan referred to in the Bible.

Answer 1

This sounds very much like The Descent, the first in a two-book series by Jeff Long. Published in 2000, it is just about (with a little wriggle room) compatible with the OP remembering to have read it in the 1990s.

From a synopsis on manga.fandom.com:

The novel follows events surrounding the discovery and exploration of a vast labyrinth of tunnels and passages stretching throughout the sub-surface of the entire world, inhabited by several species of alternately-evolved troglobitic hominids. While presently degenerate and brutal, the "hadals" had once possessed a high level of civilization, having reached the Iron Age as far back as 20,000 years ago; the novel posits that they had (rather viciously) mentored subsequent human civilizations, and that interactions with them form the basis of the historic belief in demons. The "Descent" of the title refers not only to the literal act of descending, but is also the term the narrative applies to the large-scale military-led colonization of the planet's interior that takes place in the novel.

The beachball is one of the most recognisable images from the work.

Among the artifacts made by humans were several made from them. She came to the notorious 'Beachball' made of different-colored human skins. No one knew its purpose, but the sac - once inflated, now fossilized as a perfect sphere - was especially offensive to people because it so coldly exploited the races as mere fabric.

The incident with the Chinese submarine occurs during the expedition's descent. Rather than the future though, the signal was sent from the past:

They had not yet broken camp that morning when Walker's communications specialist picked up the signals. The entire expedition had waited while more sensors were laid out and the long-wave transmission was patiently harvested...

Luckily, one woman was fluent in Mandarin. It was a distress signal sent from a People's Republic of China submarine. 'Get this,' she told them. 'The message was sent nine years ago.'