Sacredness is a key theme in the novel. Hunt portrays what lives underground as protected and sacred, as much of the history of humankind is centered around sacredness and spirituality. However, present society is drifting towards secularism — and as such, the message of the book is all about rediscovering what society has lost and the link between the past and the present, all of which can be recovered via the underground.

Red ochre is a mineral that comes from the underground that is considered sacred and was important to the Australian Aborigine community. When combined with water, it became a paint which characterized religious rituals. Drawings painted with red ochre still remain in caves thousands of years after they were created, where Hunt describes feeling a sensation of sacred power when visiting them. Humans have spent a large part of history worshiping gods and spirits in the underground and, naturally, many signs of sacred rituals can be found there. 

Hunt believes that there is more about humanity that needs to be discovered or rediscovered, and that the underground plays an important role in this. The book itself is an invitation to all readers to explore themselves and discover what they have lost, then use this knowledge to live happy lives knowing who they are — as humans. 

Throughout the novel, Hunt discusses the existence of spirits in the underground world, where they linked ancient civilizations to gods and spirits. In ancient times, caves were used to host religious rituals as people believed that spirits lived there. Mines, on the other hand, were believed to be haunted by more evil spirits, often revengeful ones. Therefore, this led to miners having to offer sacrifices before attempting to excavate any minerals from them. For instance, the Wilgie Mia was believed to be home to a spiritual being, and families have lost their loved ones through sacrifices.

The Mayans also believed that spirits lived in a chamber in the Balankanche jungle cave. When explorers opened the chamber, it is believed that they had awoken the spirits. Because of this, it had to be cleansed by shamans to appease them.