DNA is the body's master molecule. It determines what we are.
The DNA molecule makes up genes. Life is controlled genetically by a mechanism within DNA.
On the earth's surface the DNA is constantly being bombarded by ultraviolet light from the Sunwhich causes the damage to the DNA.
Normally the enzymes repair the damage. However after several decades of exposure to ultraviolet light the DNA is damaged beyond repair. Then the damaged DNA sends an incorrect message to each cell, causing it to break down and cease activity that causes a loss in the normal life processes.
This is the cause of aging and deterioration. Ultraviolet radiation affects every cell on one's body and since cells make up every organ, gland tissue and bone the effect is total.
If one spends time innthe subworld away from the ultraviolet light one will begin to feel physically better, because the DNA will not be further damaged and enzymes will not be overburdened with repairing cells.
This is the reason that Teros and Deros humans that live in the underground tunels of the earth and animal life have great size. Their cells have a chance to grow without the continuous destruction.
Terms have an average age in their world of three to four hundred years.
Terms are not able to exist on the surface for their environment has not changed for thousands of years
From the book Valiant author's Best of the Hollow Earth The search for Subterranean Species and Spaces by Valiant Thor